there will be plenty of people telling you to use an agent, but it's really unnecessary.
It's not as difficult as applying for a visa to the UK or USA/Canada etc. It's refreshingly simple. You apply for the non O retirement visa before coming to Thailand (simple). Fly to Thailand. Open a bank account. Deposit 800,000 baht. 2 months later apply to extend your stay for 12 months.
I have extended 3 times now and it's a few photocopies of your documents, passport size photo, 1,900 baht, bank print outs and an A4 form with questions like your name, passport number etc. 1 hour later you can stay for another year. Some people find that difficult...I don't.
I did my first one online 3 years ago, but it seems to have changes in most provinces. If you are between 15 and 7 days before your due date, give it a try. The worst that can happen is it's rejected and you still have time to go to the immigration office to file it in person.
If you make a record of the exact answers you provide, it's really quick the second time. Good luck.
I must admit I don't know much about the oa, as I applied for the o, but I thought it was a 12 month visa and if you time the insurance correctly and leave Thailand just before the end of 12 months, you get approximately 24 months, without needing to extend?
If that is the case, for someone planning to travel around, it might be preferable to depositing the 800,000 for the non o.
I think the obvious options are non o or oa retirement visas.
The non oa requires specific insurance and criminal checks etc.
THE non O requires you to deposit 800,000 baht in Thailand.
If you get the non O apply in the UK and then open a bank account, deposit 800,000, then once the money has been in the bank for 2 months extend your stay by 12 months and get a multiple re-entry permit at immigration. You can then extend every year.