A good deal...?? Holy chowowa! Thailand is the land of good deals...
I checked out this recommended SPA place http://www.katathani.com/index.html
in Phuket. It's around 50GBP perroom ! They do lots of packages aswell. Banyan tree looks amazing, but I dont think its worth 5 times as much.
Also Bangkok just has a multitude of stuff. Some fantastic looking hotels like metropolitan and Sukothai for around 60GBP perroom. Lots of 4* for around 30GBP too.
Are you going from Blighty or HK? I have looked at flights and internal flights are around 50GBP. Round trip flights from Hong Kong are less than 100GBP all inc.
Have a look at expedia for flights. Check out asiarooms.com for hotels.
As for things to do.......have no idea.....not been there..... yet.