What are the best visa options for a UK citizen looking to stay in Thailand for 90 days without leaving?

Get a 60 day visa ,it is valid for 3 months b4 entry .come on a 30 day visa exempt, leave the country like you said you would be doing for a month or so, and come back using you 60 day visa.

It can be used within 3 months of application

BTW, the online process is still not fully online. You can apply online, but you still need to go to the London Embassy/consulate in person with your passport. (As of about a week ago.)

James , what I do is get the tourist visa 60 days , extend at immigration for extra 30 days gives you 90 days, then border bounce for an extra 30 days , 4 months total easy mate

They are proposing to increasing the visa exemption to 45 days and make that extendable by another 45 days. Whether it will happen anytime soon is anyones guess

You can get an e-visa for 60 days and extend once for 30 days 1900b.

Easy to do and get 90 days.

If you come in VISA EXEMPT you get 30 days extendable 1 time for 30 days for 1900b.

If you exit Thailand and re enter VISA EXEMPT again it is the same.

So for example:

1st entry 30 days, leave and go to other country.

Enter again 2nd entry visa exempt 30 days and extend one time 30 days for 1900b.

You can easily get the time in country you are looking for.

Yes, a tourist visa is the easiest way to get 90 days inside Thailand.

Just because the visa application process is now online, it hasn't changed the rules. You must apply through the country you are physically in. So if you are not in UK, you cannot apply through the UK embassy online.

None of the countries near Thailand use evisa and require in-person applications. So if you want to come to Thailand first and then go somewhere else, you will need to apply based on the rules in the country you are in at the time.

Finally, as a UK citizen, you cannot get "Visa on Arrival."

You qualify for visa exempt, which means you have NO VISA at all, and are simply stamped in for 30 days. Visa on arrival means you apply for a visa when you arrive and pay for it.