Multi entry tourist visa from UK is fairly easy and straightforward to get. It gives you upto 9 months but you can only stay in Thailand for 90 days in one go. Then just hop over nearest border and straight back in, you get another 90 days. Its the simplest option if you only want 5 month stay.
You don't HAVE to leave at 60 days ,you have the option to extend ( as most people do)but you do have to leave at 90 days and re enter. If you foolishly stick to the 60 days you won't get 9 months.
Best and easiest way is to apply for Multi Entry Tourist Visa in your home country. It will give you a max stay of 9 months. But you will have to exit and renter every 90 days.