I would give Jessops a miss unless you're desperate. The big London branch at New Oxford Street will have your film but they are expensive. I've never used Robert White for film but I'm sure they'll be fine. However, just about the best regarded company for film is Mathers at http://www.mathersoflancashire.co.uk. If you give them a ring they will do their utmost to get the stuff to you as fast as possible. Get them to ship it to your relatives and it'll be there next day.
Enjoy your trip.