This is the cheapest Nexus 6 i can find. But you have to wait and go through the Standard O2 refresher cancellation procedure.
Pick the £43 a month 8GB Data Plan with £49.99 upfront, to get a £15.00 per month device plan.
(15*24) + 49.99 = £409.99
You cancel the £28 airtime as soon as you pay off the device plan.
Get the phone unlocked for free from O2. (http://www.o2.co.uk/help/phones-and-devices/unlocking-your-iphone-and-using-it-on-another-network)
When phone arrives, pay off the device plan, £409.99, and cancel the airtime.
UPDATE : It can be 409.99 if you pay 49.99 upfront with 8gb tariff (thanks to robdevil)
blue colour only
This is now on the 10GB Tariff