Hello, what type of account do you have? You can check the cost of going to Thailand by texting Thailand to 777 there is also roaming information on the recorded line 330.
For me personally I have a pay monthly tariff that has Round the world roaming but for these kind of countries are £5 per day for 3000 minutes to call back to the UK and within Thailand, plus 3000 texts plus a high amount of data.
However if you do not wish to use your phone on particular day, make sure you switch off data roaming on your phone because apps on your phone constantly use data in the background and it will trigger the £5 a day charge.
People are on different plans including pay as you go.
One of the other good options is a Vodafone Pay as you go sim. They offer a round the world bundle for £25 and offer 200 minutes,4GB data and 200 texts. This lasts 15 days. However if you get a new Vodafone pay ad you go sim, make sure you remove any active bundles on it as it swallow your credit for a bundle for the UK, I had to use their live chat to cancel it and give the credit back. You can call 2345 to get rid of it. To get this offer check the Vodafone website as you need to to text them with the tariff you want.
In the meantime check out Three's website about roaming, the costs are here and instructions. Hope this helps!
https://www.three.co.uk/support/roaming-and-calling-abroad/roaming-abroad#RoamingCosts
