Tomos has been selling mopeds in the U.S. without interruption since the 1970's. They're easy to find over here, both new and used, plus parts and service remain available. It's really the only game in town when it comes to a pure "moped."
Chinese (not be confused with Taiwanese) scooters are iffy. It's not that they're all bad, but quality varies tremendously from one company to another. Parts and service can be problematic as well. In general, the bikes being built in the PRC by companies based elsewhere (e.g., Japan, Korea, Taiwan) are pretty decent. For example, SYM (Taiwanese) is building some scooters in the PRC and they're just fine. And they come with the SYM service and parts distribution system.
I don't know what's available in the UK, so I cannot comment on them.
...Operating with impunity in the legal gray area between bicycles and motorcycles....