I would expect B&Q to carry it, but they don't know what I am talking about, and I am not sure which brand is most common here.

Online will be a last resort - any good suppliers out there?

If you mean Bacillus thuringiensis rather than British Telecom, as far as I know it is only available to licensed major commercial users.

Resin

I did find a few websites that sell "Fungus Gnat Off" and they claim that it is certified for non-commercial use. Of course they only sell it in commercial quantities! At least it has a 4 year shelf life! .

As to the mosquito dunks - I don't know how long you've been over here but you'll find out that in most parts of the UK (not counting the famous Scottish midges) mosquitoes are not much of a problem. Plus we do not(yet) have the various nasty diseases which they can carry. But then there aren't that many evenings when you bee warm enough outdoors to find out!

Suggests to me more one or the other of the following:
1. It isn't considered safe for Joe Public to use, or more likely . .
2. 99.999% of Joe Public has either never heard of it, or if they have, couldn't give a toss about using bio control rather than deadly poisons, so it doesn't sell well enough to be worth sellers stocking it.

"If the compost is well ventilated and not over watered gnats should not be a problem"

Sorry, that's not true. Soil dry enough to kill off fungus gnat larvae is too dry to support plant life, other than perhaps some cacti. And it doesn't help letting just the top get dry, as the gnats also breed in the bottom of pots, getting in through the drainage holes.

Resin