In theory you can return immediately, however no one can predict what the Immigration Officer will do, they may let you in they may not. Basically it's a 'how long is a piece of string?' question.
Easy to get in the UK. Usually issued within 2-3 days. If you're getting it based on marriage then take a copy of the front/back of your marriage cert, wife's ID card and blue house book info page. Nothing needed from Thailand to apply based on being 50 or over.