if you get everything set up so when you get out of quarantine you start the paperwork you should be fine. They need about 30 days left on your current stamp to get everything done.
yes you can switch. you'll need at least 30 days on your current stamp in order to change over to a ED visa (it can depend on the type of visa you are currently on.
if you are still in the UK, go into your bank and send a wire transfer. As long as the account in Thailand is a GBP account (make sure you have the correct routing information, or else you'll loose funds or it may be converted) the funds should be transferred correctly.