Can you not get a single entry tourist visa from Thai embassy / consulate before you arrive, that'll give you 60 days (plus extend by 30) - with a reentry (single or multiple) permit which keeps your visa alive you can come and go as you please during that 90 days.
If your doing a return flight and given your 'open' plans, I would recommend you get a ticket with flexible return date. Alternatively / depends if you can arrange your ticketing to Vietnam / Cambodia to show onward travel out of Thailand for that entry and then keep your airline ticket 'long'. [But that may cause hassle boarding flight in uk]