yes it can be extended and yes it can be extended for a year. You make a really good point that it is going to be very tight. You are going to have to open the bank account and transfer the money in as soon as you get out of quarentine.
At the moment you may get a little bit of help though the unable to travel due to COVID 60 day extension. This is due to end on the 30th May but is likely to be extended. Keep an eye on this group I am sure there will be a post if and when it is extended.
it looks like London are now issuing non immigrant O visas again. They did stop.
Just check the financial requirements that sound a little strange.
Financial evidence e.g. A copy of pension statement if the applicant is a pensioner, or a copy of 1-month bank statement showing your income from pension, or 3-month bank statement of at least £10,000
The way this reads is that you only need one month proof of income (65, 000 Baht equivalent) or you need £10,000 in your account for 3 months. This is a definite change of requirements.