Mad Max Trilogy on Blu-Ray at zavvi.de for 12.89€. Insert BXST2 to get 2€ off. Turns out to be around £8.60, depending on how you pay.
You'll get the box also at Zavvi.com for £9.99, but the code is not working here.
For Zavvi.de you'll have to create a new account but shipping will still be for free.