That is indeed a good question!
Post retirement of the 146, No. 32 (The Royal Squadron) would appear to then consist of one helicopter, presumably being renamed The Royal Flight again. The A330 is not a Detachment of 32, but indeed part of the Brize/Airtanker orbat. How the A321 fits with anyone, at a remote location and under Titan’s management is undoubtedly causing some head-scratching at HQ Air.
The rest of Northolt, exempt 1 AIDU, is largely a collection of orphans from Uxbridge needing a home. And the costs are presumably partly subsidised by Heathrow-Lite, the Biz Jet facility.
It’s a conundrum