According to Henry Gilbey the upper channel is full of the things, on one programme he was fishing between the new bridge & what looked like Clevedon Pier? He & his mate caught a shedpile of the bloody things, on a concoction of baits!
Don't know the area that well but on the Welsh side thornies used to be caught at Redwick off the ledges. On the English side you've got Ladye Bay at Clevedon, Sand Point, Weston Pier, Brean Down. No doubt there are other marks but these are the obvious ones that spring to mind.
Agree with fishingrhod on Sandpoint, Clevedon and Ladye Bay, but if you fancy a nice long day out come around to either Witches Point or Monknash beach a few have showed already this year, my mate caught a 15lb 2oz Thornie off Witches a few weeks ago plus a 12lb + Small eyed came off there last friday.