I've been stung three times in the past month with fake notes. Twice in Glasgow (one an rbs note, the other bank of England) and once in London... Went to pay for lunch and the cashier caught it with a detection pen. It had a watermark and everything.
So I've just ordered a uv detector from Amazon. There are several option under twenty pounds. Read the one and two star reviews to see which ones are fragile or you can't find replacement bulbs.
Putting notes under the light may be awkward, but I'll sweetly explain that I've been burnt and they're impossible to spot with the naked eye, so if they've got a fake it's not presumed their fault.
Shops will confiscate fakes. Obviously, I'm not willing to do that. I'll hand it back and ask if they have another one. If it's one note out of several, the assumption should be that they were fooled, too. If it's a bunch of notes, I'd assume they knew and try to get them to leave quietly.