I've been in and out of Booker for one aviation reason or another for years, and had quite a lot of contact - and occasionally flown with - what is now Booker Aviation. It's a merger of three companies: the one that manages the airport, the old Airways Flying Club (now the private flying side of the shop) and the old Wycombe Air Centre (now the commercial training side of the shop + the maintenance company).
Like many GA organisations there have been a few "issues" from time to time, and a share of strong personalities. However, overall I've always regarded the place and the company(s) as pleasant, well run, good value (for that close to London anyhow) and they tend to employ both engineers and instructors who are well above average. I also had an emergency there about 18 months ago (in my own aeroplane) and have nothing but praise for how the airport handled it.
So if the question is "would I recommend Booker for professional flying training", then I'd say absolutely yes - one of the best places I know and I'd have no problems using them for training for myself.
I thought that they were using a Technam P2006T for ME training? - but I've again heard nothing but good for the quality of pilots trained on the DA42 if that's what they're now using. I suspect that as an aeroplane it's just disfunctional enough to create good pilots without becoming unsafe (but that bit is hearsay, I've not flown one myself).