I'm doing my PPL at Wycombe Air Centre.
I work mon-fri 9-6 in central london so have no chance to fly in the week, so have been flying on weekends. Although the weekends are busier it's not too hard to get lessons booked when you book in advance, though I've usually been fine booking just a few days before! The only difficulty with weekends is that the instructors usually have 1 weekend day off - to counter this I have 2 instructors one who is in on Saturdays the other is in on Sundays.
I had my first (trial) lesson in February of this year, since then I've done 14 hours (and just went solo =] ), usually doing an hour each fortnight. I've only ever had one lesson cancelled due to weather, they called me in advance to say I could fly if I was in an hour earlier, however I could not make it.
I presume that as I progress the more solo, and away-landings may mean it's harder to book slots and more likely that weather will interfere, but so far so good.
The instructors I fly with are both very good, and the others I have met are also very friendly people and I presume teach just as well. They are very professional and keep you on your toes but at the same time very easy to get on with and fun to fly with.
The airfield is very busy with gliders and helicopters - and has a tower so you have to learn the R/T from the start, also there is a mix of both grass and tarmac runways to keep things varied. (not sure how these points compare to Denham.)
The circuit on rway 27 takes about 10 mins due to noise abatement zones (and seems to be the rway in use 90% of the time!), 06 circuit is a lot quicker. I don't know if denham has a quicker circuit, which would mean you get more touch and goes in an hour lesson. (though the 10 min circuit is good when you start circuits as you get a chance to think things through..)
I have no experience with the ground school but it's timed so I would be able to attend if I leave work a bit early.
Obv. no experience with denham but those are my thoughts about WAC. I'm liking it and if you are in a rush to get your PPL you should be able to cram in more hours than I have done. (could probably have done 45 by now if I'd been doing sat & sun flying every weekend, or more than 1 hour per day)