Each entry will get you a 90 day stamp. It’s valid for a year so every time you enter Thailand whilst it’s still valid, you’ll get stamped in for 90 days.
There's nothing scary about it really. Don't check in until you get to the airport. They'll look in your passport for a visa. You haven't got one. Then show them the printed out ticket out of Thailand within the time frame allowed and they'll look at it, check you in for the flight, and you're set.
If it were me, I'd buy a cheap ticket out of Bangkok to say Kuala Lumpur on a low cost airline about 25 days after you arrive in Thailand. That will be enough to get them to let you board the flight in London, once you've done that, you're fine to follow the guidance offered above re extensions, border bounces etc