there is a special lead-lined bag that you can buy to carry your film in.
It will protect it when it goes through the hand luggage xray machine.
Usually then they'll open the bag to see what's inside.
When we used to have a camera with film, we always kept the film in that special bag. I assume it's still sold (despite the proliferation of digital).
Even if you put it in your checked luggage, you should still use the bag, as checked luggage is also xrayed (at least in the US, i assume also in the UK)...
Of course, then the TSA people in the US will open your luggage.
Personally, i'd put it in my hand luggage, in that special bag and then have the security people hand search it.
