Not made in China?

[SubOnCotton]

I am sure if you really wanted to have your mugs made in UK you could, but you'd likely be looking at more than £1 more than imported ones, unless you were wanting substantial quantities.
There are still some UK manufactrers of mugs - Emma Bridgewater would be the first that comes to mind - not sure how much they'd be interested in making blank mugs for you. Then Listawood's website say they do mug coating in the UK, so in theory what you want is possible, if you're willing to pay for it, and then pass that costings onto your customers.

I have no idea how they are made, but I would assume that it's a labour-intensive process. I'll have a look.