London Heathrow airport offers the cheapest routes to Bangkok, from ฿20,484; whereas London Gatwick airport is typically cheaper if you're flying to Bangkok, from ฿21,156. It's best to make sure you are flexible on dates to find the cheapest possible flight options for your journey.
Here are some of the cheapest flight destinations from United Kingdom:
Fly from London to Bangkok from ฿20,484
Fly from London to Phuket City from ฿23,708
Fly from London to Koh Samui from ฿26,663