As long as you apply in UK with a UK passport you can just upload ID-page of your passport one more time. Also drivers license, utility bill or bank letter with name and address.
Generally you just need the funds in your account at date shown as an international transfer, unless you apply at Jomtien immigration. For the extension of stay you’ll need two months seasoning.
You’ll get another 60 days upon your return and you will probably be able to get two 60 days visa exemptions if you don’t do any 30 days additional extension and you can prove onward tickets. But things have become quite strict now. The safest option is always to get a multiple entries visa if you’re going to do several entries within a short periode of time. A six months multiple entries tourist visa will easily secure your whole trip with a visa. The decision is yours to take.