I found a couple of other useful resources; this (
https://www.apexdyna.nl/en/the-right-timing-belt/) is a company based in Holland (so it'll have a Euro perspective) but seems to confirm that AT is good for my needs, and that GT is better known in the US market.
The Reprap wiki (
https://reprap.org/wiki/Choosing_Belts_and_Pulleys) advises GT2 over T belts; which makes sense, and also perhaps confirms my guess that the better availability of the GT2 and GT3 parts in the UK is down to the popularity of 3D printers.
Given the relative lack of GT5 and GT10 here in the UK I'll proceed with AT. I've not found a suitable ATL belt yet (which I understand works with AT pulleys, and is even better than AT belts), but I am working my way through the various equations in the doc posted by Peter (which is proving very useful).