If a seller chooses to allow shipping via the GSP, they can also provide secondary shipping services (e.g. Royal Mail in the UK, USPS in the States).  It is up to the seller to provide shipping options and they don't have to if they don't wish to.  There's no 'lock' as such.  As I have said so many times, I have been a winner on a large scale with the GSP, having hundreds of kg of items shipped to me at a small fraction of the price that they would have cost me if I had them shipped via other means.