A great deal depends on which Country issued your passport. People in quite a few Countries now have to apply for an electronic visa via the Thailand E visa site on line.
Brandon is correct. You will have to apply for an electronic visa. You should register on the Thai e visa site and they will tell which Embassy or Consulate to apply, If UK has more than 1 office like a lot of Countries do.
A great deal depends. For example on your check in for the flight they may well ask you for an onward ticket but not always. I was denied the first time but went out and got an onward ticket and it worked fine. The second time I was denied in Japan where I had entered on a return business ticket but they said and rightly so that I had to have an onward ticket so I showed them the one I still had from before, which was out of date. He didn't didn't realise that. I told him that a UK citizen didn,t need an onward ticket from Hong Kong and I was right. He didn't like it one bit but I got on and had no trouble getting back into Thailand.
You have to leave on the passport you came in with. You can enter another Country using any valid passport as long as you meet the requirements for that place.