"Only English groups" list. These are ones you can't stop once they start playing. Mostly out-of-control keyboards!
1) Soft Machine vols. 1-4 & live from Wyatt period
2) National Health - Of Queues and Cures
3) Gentle Giant - albums from around 75-78; too difficult to pick one
4) Quiet Sun - Mainstream
5) Genesis - Lamb Lies Down; Seconds Out
6) Bruford - One of a Kind
6) Strawbs - Ghosts
7) Brand X - first lp, Moroccan roll and parts of others

Arzachel
9) The Nice - organ stuff from around 68-70
10) Fripp - League of Gentlemen & Red
11) Caravan - good quality live (eg Songs for Oblivious Fishermen)
I've probably forgotten some ...
And how about a short list of groups that haven't aged well or are over-rated (ok, I'm likely to offend some people - sorry about that!):
1) Yes & Wakeman
2) ELP
3) Marillon and nearly all neo-prog
4) American prog from the 70's
5) Van Der Graaf Generator
6) King Crimson revivals
7) Fruupp, Enid and other light symph prog

Alan Holdsworth (except UK #1 album)
9) Be Bop Deluxe
10) Camel