There seems to be a voucher glitch on the Play.com site (get what you want, use your imagination).
If you go for the 2 for £18 on Blu-Rays offer and then apply the £5 off voucher, it still works letting you have the 2 films or any other of the other films in the offer for £13.
That's £6.50 a film + 4% Quidco on top.
The voucher for Play.com says you have to spend over £50 to get £5 off, but it seems to work on most things I have tried down to items for £14.99 or even less.
You can only use the voucher once I believe unless you create more than one account or maybe use more than one email address.
Voucher code:- PLARBS09 (thanks to youremove for posting it in the voucher section).