There nothing to worry about you got your tourist visa just go they won’t stop u boarding with a setv and all the rest can be worked out along the way border runs visa runs etc it’s all part of the adventure have a good trip👍
The only issue as someone said will be checking in at airport in U.K. sometimes if your flight back is in 45 days they might not let u board as u will only get a thirty day stamp wen entering.....2 options get a tourist visa or booke a cheap flight for say a thousand baht to somewhere like Cambodia within the 30 days so u can show that when boarding in U.K. as for the other thing a 30 day can be extended at Imigration for another 30 day but that’s not the problem the problem is boarding in U.K. good luck