You Do Need power plug adapter while traveling from Thailand to United Kingdom (UK).

Here is Why You Need An Adapter

The outlet and plug type used in United Kingdom (UK) is G and in Thailand are A, B, C and O. This is how they look like.

Thailand

In Thailand, outlets of type A, B, C and O are in use.

Type A

Type B

Type C

Type O

United Kingdom (UK)

In United Kingdom (UK), outlets of type G are in use.

Type G