Hi Mike,
go to the MYA web site and there you'll find information that might help. You can get numbers and other items from model yacht specialist firms, or do as I've done and make a stencil of you desired numbers and lettering (computer word packages actually have a format for stencil or outline), then lay flat on the sails and lightly spray with fabric paint, making several light coats.
The placement can be found by looking at images of racing yachts either on trhe MYA site of just google images model yacht racing. I don't think using GBR is a problem at all.
If your yacht is a specific class, again as defined by the MYA, then it will have class symbols that have a specified prefered position. Again, I'm assuming that as you have "numbers" then it's registered to a class and been allocated the number, after measurement etc.
Kimosubby.