The actual Answer is...all, then again...none.
Director, Operational Requirements turns Defence Policy into aspirational inventory. So: when close-up to Army was an RAF Task, he specified Observation Posts and assault gliders; after 1957, Army Air Corps., he did not.
His Shopping List must then find its way into Long Term Defence Costings, which must first be Approved by Heads Army+Navy, before they all try to get politicians to extract funds from Treasury.
Surmounting all that a Tender Specification is prepared to go out to industry. Which? Who? Where? Bespoke or Commercial Off the Shelf? Seldom is RAF's need truly unique, so often someone's imminent kit would meet a high % of the Spec.
So, politics: Protect domestic jobs...by taking on a development schedule which may not run smoothly? Or fork out hard cash to export jobs, import (?proven?) kit? Ah.
Before 1949, when Alliances were erratic, consensus was to try for self-sufficiency. After NATO became effective, inter-/cross-operation became an Aim. That would lead in logic to 100% US inventory, not due to quality, but to scale of US own demand.
So: to simplify: end-User Commands would he very happy to operate the same kit as US, accessing parts, tools, manuals. Treasury would like that too, as US can be, ah, flexible, on price/payment. But local employment would be good. So: offset.
In modern times: Belfast, Belvedere, Argosy, Puma, Gazelle, Jaguar, Harrier, were imposed by politicians to meet ORs which RAF would have preferred to be met by: leased C-133, more S.58 (preferably from Sikorsky), C-130, USArmy solution, US Army solution, F-4C, F-4D.
User logic would have been same as RAAF taking F-111C, not TSR.2: if US is churning out (F-4s: 75 a month!) and deploying them on stores/training/repair facilities world-wide...what's not to love?