I am sorry to say FC that in a free market economy you have a choice as to who is going to screw you in these circumstances but a choice not to be screwed is not available....
You can bring travellers cheques. Has the advantage of security if you should lose. However, if you bring GBP you will get charged for them & probably screwed on the exchange rate when you exchange them here.
If you bring a foreign currency you will get for free, but screwed on exchange rate when you buy.
You can bring cash GBP (there are limits on cash, but I think it is something like 10,000 euro or equivalent) but you will get screwed on exchange rate here when you convert.
You could bring baht (noting limits above) but will certainly get screwed on exchange rate in UK - SAGA - the 'pensioners' club are well worth a look at if you are going this route, can see on line & do offer a far better rate (though still a rip off) than the big banks.
Credit cards are, as you noted, now being charged an extra premium by Thai Banks & are often at a shocking exchange rate.
Transfers to Thai Banks involve a fee to do (about 23 GBP a transaction) & can involve a **** exchange rate as well....