If you transit through Bangkok *without* entering Thailand then a single re-entry permit will be all you need (when you actually re-enter on your return from the UK), however, if you don't transit and instead actually enter Thailand then you'll need another re-entry permit.
yes an Metv allows you to come and go. A regular visa also allows that *if* you pay for a re-entry permit before leaving Thailand and assuming your visa has validity for your return dates.
Personally I'd just use single entry visas. You can use them more years and years with no restriction. Whereas the 6 month metv requires significant hassle.
the 6 month METV has a validity of 6 months from when you first use it, you can stay 60 days per entry, you can also do one 30 day extension per entry, however the optimal usage is to leave Thailand and re-enter just before the "valid to" date which gets you a 60 day entry just before the visa expires, then do a 30 day extension on that final entry. This gets you almost 9 months stay here in Thailand.
There has never been a 12 month tourist visa. The relatively new METV (multi entry tourist visa) has only ever been valid for 6 months. With careful border bounces you can get almost 9 months out if it, but it is still a 6 month visa.