Firstly, a big welcome to the forum here, Lady Vendome, from a Kentish valley not so far from London!!
I'm pretty sure I've had decent gyoza in london once or twice at restaurants, but I'm having a bit of trouble remembering exactly where. I'll try to rack my brains a little more.
In the meantime, what kinds of places have you tried? I was very disappointed in Taro (but then everything in there disappointed me, and I shan't be going back!) and Osatsuma in Soho was OK I think, but not great.
The best gyoza I've eaten were cooked at home by Japanese friends. Perhaps you could try yourself? The recipe's really simple, and you just have to pick up some ingredients from an Asian food shop, like the gyoza 'skins'.
Good luck, please let us know how you get on!!