I have used them for an FI revalidation but cant really comment on the IR training side of things. I have generally only heard good stuff about Wycombe, but the issue with Wycombe is there are no instrument facilities on the airfield, so the IR training flights will involve transit time. If you are training somewhere where there is NDB and ILS on the field than every approach back to base will be another instrument approach. No matter how good a school Wycombe is every other approach on your IR will be a VFR approach back to base; that will limit the efficiency of the training. At 400 plus an hour for ME IR training those 20 minutes transits will soon add up.
That said there aren't many modular training providers left. Also consider what type of flying you want to do afterwards, there are a couple of operators out there who wont take someone who did the initial IR on a Twin Star because of past problems with Twinstar trained pilots trying to transition to analogue.