Hi Hovis,
A PERSONAL OPINION
From personal experience, we always use UK sterling or debit card (VISA). ATMs are plentiful (in towns), easy to use and you usually get a good rate of exchange. In many visits we have not had a problem.
We still find that the Nationwide Flex card give one of the best rates - although they now pass on Visa charge, there is no other charge. Travellers cheques can be costly. For goodness sake don't take UD $ cheques or you will be hit with two conversion charges - GBP to USD when you buy the cheques and then USD to Thai Baht when you change to local currency. Travel agents usually recommend USD or travellers cheques, mainly so they get a commission!. TCs can also be a nuisance to get changed - hotels may have a changing system, but you always get a lousy rate! (as indeed for cash).
When you arrive in Bangkok, ATMs and money changing is simple at the airport. You will have no trouble taking in the amount of cash you describe - you can take in far more.
Just be careful that the amount of Baht you receive when you change currency, tallies with what you should have received.
Most hotels expect to take a credit card impression when you check in. This is quite normal and once again we have had no problems. If you are sensible and don't let your card out of your sight or use in rinky dink places (just as in UK), you shouldn't have any problems at all.
Take more than one debit card, if you anticipate using these, as all card companies seem to be clamping down on unusual transatctions. Let your bank (and credit card company) know that you will be in Thailand during the specific period.
A mix of good
clean UK banknotes (not Scottish) and cards should provide you with the best exchange rates.
The choice however must be yours.
Enjoy Thailand - we do!