Can I stay in the UK for 6 months if I have a single re-entry stamp to Thailand, and how does the 90-day reporting requirement work if I'm abroad?

If you leave Thailand before you are due to make a 90 day report don't worry about, your next 90 day report will be due 90 days after you return, the day you return being day 1. You can stay outside Thailand with a re-entry permit for as long as you want as long as you are back to renew your extension of stay

You'll need to apply for an extension before you leave since your extension ends in March but you're leaving in April.

Then you get a re-entry permit and you can return any time until your new extension ends.

There's no such thing as a 90 day stamp. 90 day reporting is only for reporting 90 consecutive days inside Thailand. If you're not in Thailand you cannot report being in Thailand for 90 consecutive days.

You could in theory stay in the UK until March ‘25. You just have to enter before your stamp expires.

When you leave Thailand the requirement to report your 90 days stops and restarts at day 1 on re-entry.