Land in Bangkok Sunday for 4 nights then ko samui for 8, what is the current exchange rate, just to see if it is better out there or buy here in the UK?
You can see the current day to day rates for all the Thai banks and major independent money exchanges at https://daytodaydata.net/
Use that to compare with best rates collected or delivered at home at https://travelmoney.moneysavingexpert.com/ to determine whether you change any before leaving UK. The difference in Thailand now is not as great as it used to be.
Assuming of course you can get to a SuperRich or other private exchange with no great effort or cost from where you are staying, If you have time you can get better than bank rates at the private exchanges at the airport link level in BKK airport on arrival.
Whilst it is still true that the best rate in Thailand is better than the best in the UK the days of 3 Baht plus better on the pound are long gone these days you will be lucky if you get 1 Baht more in Thailand for example today the best in Thailand (according to the day to day site) is 44.0 Baht to the pound whilst the best in the UK is 43.2 in my opinion it is still a good enough difference to make changing in Thailand worthwhile providing you will have easy access to an exchange that gives somewhere close to the best rate.
Edit stay well away from wherever ChrisThom works even Tesco are offering close to 42 Baht to the pound.Edited: 26 March 2018, 10:18
The £ is strong at present, because (stated on business news today) the dealers are expecting a rate rise in May. We do not go until 19th April so I may wish to wait.
Dilemma. Will the rates rise ..Yes/No/Yes/No? Yes/No? Maybe?
Buy now...Yes/No? Yes/No? Yes/No? May be ? Perhaps Yes? Perhaps No? Humm gaining 100 baht for all this stress.
Why worry over the price of a bag of chips from the local Fish and Chip shop.
I have a degree in Hindsight!