I think the UK thrash scene is appropriately rated. While there is no doubt an under-appreciation for the sheer number of bands out there, most of them just aren't that good. I think beyond Onslaught and Sabbat, there are few bands you'll find people feel strongly about. Even those two don't do much for me compared to the German/American bands. The only band from this scene that stands above the crowd to me is Xentrix, but they're sort of like a Metallica/Testament-type clone, without too much unique going for them. Slammer is another one that is perhaps a bit better than the others.
The problem that this scene has is that a lot of the bands are pretty standard in what they do. There's nothing wrong with that, but playing Metallica-styled thrash is really hard because it emphasizes songwriting over speed and aggression. There's nothing wrong with this (in fact, I prefer it when done well), but these bands just aren't that good at songwriting (in general).
Sabbat rightfully gets a bit of recognition because they do things differently. Onslaught was also a bit different, though I think In Search of Sanity was by far the best thing they've ever done, and was more in the power/thrash realm that bands like Heathen and Flotsam & Jetsam did so well.
Also, I'll add a few more bands to the discussion, in case anyone is looking for more: Xyster, Metal Messiah, Snyper, Re-animator, Virus (these guys are a bit more interesting, particularly on Force Recon). It's also worth looking into the label Mosh Tuneage, who is doing everything they can to re-issue every long-lost UK thrash album: http://moshtuneage.bigcartel.com/