Not sure how they compare price-wise, but Wet & Wild at Hamble Point did some graphics for us, from a design by SWMBO, and made an excellent job of it.
Just look in the yellow pages for your nearest signwriter. Tell them it's for a boat, and they'll use UV resistant vinyl. They are all computerised these days, so almost infinite range of fonts and graphics. For this sort of thing doubt they'd charge more than £15 per name and probably less
Thanks everyone. I've got several contacts to try now. Boat Names are quoting £1 per letter which seems like a pretty good deal. And a friend has recommended a non-boaty graphics company as Brendan suggested so I'll see what they charge without "boat added tax".