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.