Well, I did mine effectively full-time. It took me a little over thirty flying days (about 45 days total) due to a lot of really poor weather as well as early sunsets. TG is busy enough that the guys know what they are doing but quiet enough that you can effectively just dictate when you want to fly. I aimed to fly for 2 - 3 hours a day but it worked out around 1.5 hours per day in the end. In the beginning two hours of flying is pretty exhausting, especially when you are trying to take in huge amounts of information at the same time as actually getting used to flying the plane. I would say that if you stood a chance of doing the PPL as quickly as possible with anyone, it would be TG.
Just a hint, when you do your training don't make the same mistake I did and let your exams slide until the end. I ended up doing about 5 exams in the last 2 weeks which wasn't a lot of fun. Sure you aren't as dumb as me though...
As to the PA-28 question, it is very much advisable to train on TG's (£15 / hour cheaper) C-152s and then do a one or two hour conversion (depending on your aptitude) onto the Warrior towards the end of the course. You will probably end up with an hour or two to spare towards the end which you can use for this purpose. That way it wont cost you more than £30 to convert, rather than the £500 extra it would cost if you did the whole thing on the PA-28.
Oh, finally, one definite advantage to training at manston is the traffic. You will probably see quite a lot of heavies if you use TG. It can be a lot of fun to be flying amongst 747s and a340s. Although an a340 doing touch and go's for 3 hours became a bit tiresome one day (a rarity thankfully).