I've done this. Add about £40 to the seller's bill. You will have to pay V.A.T. and ParcelForce charge about £8 for collecting it and arranging the delivery (even if you pick it up from their depot).
But you don't need to pay any import or custom duties. The Sony Reader counts in the same category as a pda, so no customs duty is payable..
It's all worth it , in my opinion. I've saved a small fortune by curbing my Amazon habit. And I'm slowly reclaiming my storage space by converting public domain books.
See our book uploads section for a shed-load of free books in lrf format.