Sadly there are few films set on British long distance paths - I guess midge bites and soggy socks are no comparison for wrestling bears or falling off huge waterfalls in Alaska. Yep, movie makers tend to focus on the more hardcore trails: the Appalacian Trail, The Pacific Coast Trail, that kind of thing. The only exception I know of is Downhill the Movie which is based on the Coast to Coast. Instead of hiking through 100 mile wildernesses, you get the British equivalent which is having a tantrum about the rain and making the trail into a massive pub crawl. Ah well. Here they are anyhow: rest assured you won't be dealing with anything as challenging as these characters go through!
Martin Sheen's film set on the The Camino de Santiago, a very popular long distance trail in Spain.
Reece Witherspoon stars in this popular film about a woman walking the Pacific Coast Trail
Comedy: 4 blokes in a mid life crisis walk the Coast to Coast. It's pretty blokey (believe me there are no women stripping off to their bras on the Coast to Coast!) but worth it for the British scenery and the depiction of walking the trail.
I haven't seen this but it looks interesting...
Coming in 2015 this is the movie of Bill Bryson's book about walking the Appalacian Trail! Can't wait!
Here are some of the best documentaries about specific trails in Britain.
Note: these are links to DVDs in Amazon.co.uk - they may not work in other regions. The links are affiliate links.