There is a UK law that restricts shipping of 'drug paraphernalia'. After going round and round with the GSP help desk, they finally told me that Pitney-Bowes is calling smoking pipes 'drug paraphernalia'. That is the loophole that they use to steal them and then re-sell. I had 5 pipes stolen by them last Fall, and when they showed up on ebay again, I contacted the seller ( thefactoryoutletstore), described to her what had happened, and tried to buy them. What did she do? She blocked me.
Now I avoid UK sellers who use GSP, but on two occasions I really wanted the pipe, so I did what you are suggesting, and they shipped Royal Mail, skipping GSP.