As you mentioned, it is very rare for Thai Immigration at an air entry port to ask to see an onward ticket. Based on your scenario, you should have no difficulty with Thai Immigration - if, during your initial stay of 30 days, you go to an Immigration Office for a 30 day extension and they want to see your onward ticket, your departure will be within the next 30 days. So no problem there.
As mentioned, the likely problem, and this depends on your airline, will be in boarding in the UK for travel to Thailand, as they may require your onward ticket show departure within the 30 day permission to stay granted upon entry for Visa Exempt arrivals. As I understand it, there are websites that for a small fee, cab provide a "ticket" for that purpose (not a real booking).