i don't think you can, i've been caught out by this before. i thought i was using my voucher but ended up paying out of my account for the seller items and voucher for the amazon ones
From the Amazon website:
"Amazon.co.uk gift certificates and gift cards must be redeemed on the Amazon.co.uk website or the affiliated website Javari.co.uk, towards the purchase of eligible products listed in our online catalogue and sold by Amazon.co.uk or any other seller selling through the Amazon.co.uk platform".