STOP PRESS!
It turns-out, after re-analysing and seeking professional opinion on the weather forum (by that, I mean the impression of someone who has made a career in meteorology), that I wasn't far off after all. I hereby renounce my hasty decison to cease posting in this thread.
To clarify, this morning's models indicated an earlier and more rapidly-developing switch to mild south-westerly winds than I suggested yesterday. It now appears that this will happen from around Tuesday/Wednesday of next week, with temperatures rising first in the south-west, but the unsettled conditions (wetter and breezier) drifting down from northern Scotland. This set-up may last for quite a while; if this winter's background state in terms of global patterns was more average, I'd say that with a degree of confidence, but such background signals are contradictory and unusual this year, so it may be that it lasts for 5-7 days only, with colder conditions possibly following shortly after mid-month (from around the 17th or so onwards).