I am not at any groundschool as yet, but have done a little rooting around. All the ones you have mentioned are reputable (please, no harsh replies, I am only trying to help!).
But, my choice will be either London Guildhall University or ATA.
I did look at the highly recommended Bristol, but they don't do a full-time course which is what I want to do.
Best advice has to be visit as many as you can on your shortlist, then collar a student and ask them how their treated. I believe this is pretty universal advice>
Best of luck!!!