Harry Potter 1-4 (Blu-ray) for $15 each. All 4 are region free as well.
You should be able to order 2 at a time & still avoid paying customs as its $30 for 2 (around £15- or £20 inc p&p).
If you're worried about customs then £11 each delivered, which is still a pretty decent price.
The best Harry Potter film is Prisoner of Azkaban & looks amazing on Blu-Ray, & is well worth buying.