Yes. Monzo won’t add fees, the amount will be converted from Baht into GBP using the MasterCard exchange rate. Just make sure the amount when it comes to payment is in Baht.
You can use the MasterCard calculator here: https://www.mastercard.us/en-us/consumers/get-support/convert-currency.html to find out a rough idea of how much it will be in GBP. Put 0 for bank fees as Monzo doesn’t charge any.
The exchange rate constantly fluctuates so the amount may differ slightly when you eventually pay. The transaction in your Monzo app will show an estimate in GBP but this may change slightly when the payment settles according to what the MC exchange rate is when that happens.
Hope that helps?