It seems like a 60 day, double-entry visa ( 25 GBP per entry, so 50 GBP in total ) would address any potential issues?

This would give you two entries, 60 days each, and each can be extended 30 days inside the country for a total of 180 days.

So while you might use one entry on the first short stay, the second entry would be good for 60 days, and could be extended another 30 days, to cover your two-three month plans.

I assume you have a British passport?

You could forego the visa in the U.K., use a 30 exemption for your first arrival, then get a 60 day tourist visa in Sri Lanka.

http://www.thaiembassy.org/colombo/en/home