Compatibility summary
You will need a travel adapter in Guernsey; plug types used there differ from Thailand.
Plug comparison
Thailand plug types: A B C O
Guernsey plug types: G
There is no common plug type between Thailand (A B C O) and Guernsey (G). You need a travel adapter.
Voltage comparison
Thailand: 220V, 50Hz
Guernsey: 230V, 50Hz
Voltage difference is within a range many devices tolerate; check your appliance label.