I am not sure how OP is hoping to save on taxes and duties. Re-shippers would declare the purchase price in their shipping documents anyway. You can only save on high shipping rates that GSP has.
Unless you reship via an individual who agrees to declare as a gift.
The GSP shipping rates are reasonable, in fact they are cheap.
But the duties and taxes that Pitney Bowes aren't really taxes and duties. As a VAT registered business in the UK I should be able to reclaim the taxes once the item enters the UK, but Pitney Bowes don't issue the invoices to allow me to do that. What they do with the taxes they collect is open to question - many questions...
I'm not trying to avoid legitimate taxes here, because I could normally reclaim them all. But GSP makes me pay them in a way that makes it impossible for me to reclaim them.
Pitney Bowes are thieves.