I found some further information re C.E.F. men transferring to the B.E.F. upon receiving commissions - this only covers the first contingent though. The following excerpts are from Col. A.F. Duguid's "The Official History of the Canadian Forces in the Great War; August 1914 to September 1915" (courtesy the Archive CD Books Project, link).

(pages 140-141)

Reductions of strength in the same period numbered 1,144, of whom 131 were officers. The latter figure comprised........31 were struck off strength on appointment to the British forces, of whom 27 were posted to the New Armies.

The excess of officer material in the Contingent and the shortage in England were recognized by Lord Kitchener. The matter being urgent he, on 5th November, suggested to General Alderson that 300 Canadians might be supplied to the British forces.

The 1,013 other ranks may also be categorized:.........The discharge of Canadian other ranks on being given commissions in the British forces accounted for almost one-quarter of the total reduction in the strength. The Canadian Government acceded to the request of the British Government that applicants from the C.E.F. might be given commissions in the Special Reserve or Territorial Force or temporary commissions in the British Regular Army, subject to the commanding officer's recommendation. Two hundred and sixteen were commissioned in the New Armies, colloquially termed "Kitchener's Army," now in course of being raised in the British Isles; twenty six were commissioned in other British forces, including the Royal Navy and Indian Army. Thus the total number of Canadians, including those already commissioned, who were transferred as officers to the British forces in this period was 273.
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The following footnote appears in the appendices volume, pages 140-141, and shows there appears to have been some "horse trading" between the War Office and General Alderson when it came to allowing C.E.F. men to receive commissions in the B.E.F. and being able to recruit Canadians in Britain:


NOTE ON RECRUITING FOR CANADIAN CONTINGENT IN ENGLAND, 1914-15

The question of recruiting in England to complete establishment was raised on 29th October 1914 by the O.C. Royal Canadian Dragoons, who asked permission to enlist twenty-three " good men" who were then offering; approval was given by the G.O.C. Canadian Contingent. On 12th November the War Office reversed this decision:

Enlistment in the United Kingdom for the Colonial Contingent cannot be allowed.

This was published in Salisbury Plain Orders next day and at the same time General Alderson protested to the War Office that:

. . . . as numbers of both men and officers are being permitted to transfer to the Imperial Army I should be glad if I might be permitted to enlist bona fide Canadians who may be at present in the United Kingdom, or who may have come over from Canada for enlistment in place of those transferred.

Prior to this certain units of the Contingent, including the 17th Battalion, had advertised for recruits in England.

In reply to General Alderson's protest, the Army Council said that the taking on of personnel of one of the independent detachments of Canadian volunteers (Elliott's Horse, see Appendix 196) had been authorized as a very exceptional case at the personal request of the acting High Commissioner for Canada. And, further, the Council pointed out that this was not to be taken as a precedent, that no other men were to be enlisted in England, and that the policy of the Army Council and of the Canadian Government was that all enlistments for the Canadian Contingent should be carried out in Canada.

On 8th December General Alderson in a letter to the Southern Command again protested, saying that, in response to a request of the Military Secretary, he had forwarded from the Contingent no less than a hundred applications for commissions in, and had made no objections to the transfer of officers to, the Regular or Territorial Army. In accordance with War Office instructions he had been refusing applications from bona fide Canadians to enlist, and he again requested that he be allowed to enlist such men, up to the extent of the men discharged on appointment to commissioned rank in the Regular or Territorial Army. " If this is not allowed, commanding officers can hardly be expected to look with favourable eyes upon the appointment of men to commissions."

Three weeks later the Army Council modified their previous decision: the proposal of General Alderson was not approved but special cases might be submitted to the Army Council after very careful investigation of each. When, in accordance with the above, General Aiderson on 12th January submitted to Southern Command a list of fourteen bona fide Canadians it brought a reply from the Adjutant-General that:

Lord Kitchener has always been opposed to your recruiting Canadians in this
country for fear of our being inundated with men from the Colony, and by so
called Canadians in England.

The very suggestion that higher rates of pay could be obtained in England
than our regulars get, would have a very disturbing influence on our recruiting.

Lord Kitchener suggested that vacancies should be filled from Canadian reserve units, so that trained men would be secured instead of recruits; but fifteen special cases were subsequently approved.

The question was re-opened by Br.-General Carson on 25th May 1915, when he wrote to the Secretary of the War Office asking that enlistment of "strictly speaking Canadian citizens who may be in England " might be permitted. The Director of Recruiting a week later wrote to the Secretary of the High Commissioner " with reference to Mr. Carson's letter to Sir Reginald Brade . . . . that under the present circumstances the War Office had no objection to such enlistments " and at the end of June a ruling was given that attestations in England must be restricted to—

Any man who can satisfactorily prove that he has not only been a resident of Canada, but that he is now domiciled there, i.e., that his home is established there, . . irrespective of the length of time he has resided in Canada. This refers to British subjects.

Under this ruling, in the months of June to September 1915, inclusive, 123 recruits were enlisted in England.

Shortly before the Division left for France it was found that there was a shortage in the Artillery of about thirty artificers, farriers, shoeing-smiths and saddlers; General Aiderson therefore on 12th January requested that authority be granted for their enlistment in England; the request was acceded to and the enlistment of non-Canadians carried out to this extent. One S.M. Instructor R.E. attached to the Canadian Engineers, was taken on strength of the C.E.F. on 10th February 1915.