What visa options are available for a UK passport holder traveling from India to stay in Thailand for 3 months?

I would think you will either be able to get a covid extension or if they end them things will open up to do a border run cheap flight to Singapore for a week

So after reading all the comments and thinking back on the situation, there's a chance you can apply for the new "stickerless" tourist (TR) evisa from the UK embassy. Because you no longer need to mail anything in and they just send you a receipt to print and take to the airport, this may be a viable option. Then you only need to apply for a COE from India based on a tourist visa. The stickerless visas are less than 2 weeks old at this point so it's an interesting and untested method.

Don't get a visa exempt entry, that only gets you 60 days including the extension, a TR60 tourist visa gets you 90 days including extension, from there you could transition to an ED visa for another 90 days.

If you don't want to do retirement, which would be the easiest/most legit IF you can meet the requirements, you can come on a tourist entry. This can usually only be extended once whether visa exemption or visa, but there have been ongoing Covid exemptions since this whole thing started. They keep saying these are ending but they have not ended yet. So you could hope that continues to be an option; they are going to end at some point but maybe borders will be open when they do.

Another alternative, when you do get here, would be to sign up for an education visa. Together with special in-country conversion fee this might cost you around 50-60k, including course costs, but would get you a full year.

No, you cannot. Thailand is closed permanently.

The e-visa website tells you if you can apply for various visas and from where. You might be able to get a Single Entry Tourist for 60 days extendable for 30.

You can't get a visa on arrival on a UK passport but you can get a 30 day visa exemption.

I don't know if you can apply for a 60 day tourist visa from India but if you can that will give you 3 months with the 30 day extension.

If you're spending nearly 50% of your time in Thailand then you may want to consider getting a more appropriate visa instead of relying on visa exemptions/tourist visas though.

If you’re over 50 you could apply for a non O visa based on being 50 or above (sometimes referred to as a retirement visa) that would get you stamped in for 90 days.