<<If you are really worried about airport x-rays, you could purchase your film in England, and even get it processed there before returning home. This might get expensive with the VAT (tax), but at least you will have peace of mind.>>
There is a fairly decent camera store on the east side of Tottenham Court Road, quite near the Warren St. tube station (near Euston Road) where Fuji could be had, with or without processing, more cheaply than most other locations in London. John is wise in suggesting this option. Fuji processing in the UK is also an option, but be aware that it can take a week or two, unlike the overnight turnover here. Under certain circumstances, VAT is refundable, so keep those receipts if you are there for any important length of time. I forgot to mention Kensington Olympia as another tube-accessible station where a lot of freight activity can be watched. Ken --> SPORRS: Serious Photographers of Railroad Related Subjects
