Well if you are worried, then buy it in London and have it processed there too. It's not like you are going to some third world country where you can't buy the films and have them processed before you leave.
Actually this is not the best procedure if you're a professional and image quality is your livelihood. If your image quality is important you need to test your film, especially if it is color, and have that test processed in the lab where you intend on processing all of that film prior to shooting images of any importance. Just buying your film over seas and giving it to some untested local lab, even in a major city, can be disastrous.
That is a bit melodramatic IMO. Disastrous? An inconvenience that will require some minor color, and perhaps density, correction at worst, and even then only a time and money issue with transparency film. The goal is to have all films from the same series processed similarly more than it is to have the films from one series perfectly match the films from another series. So as long as you process all the film from the same series the same way, you will be fine.
"Truth and love are my law and worship. Form and conscience are my manifestation and guide. Nature and peace are my shelter and companions. Order is my attitude. Beauty and perfection are my attack."
Is Silverprint not a Pro lab where they will do snip tests on each roll of film if you pay them to do it? Again, we are talking about London, a very large technologically sophisticated city in the modern world. We are not talking about a bush plane ride to the middle of nowhere that doesn't even have electricity.
Give me a break!
If there are a worry about Xray exposure (real or not), then maybe this thread should have turned into:
"where can I buy XYZ films for a good price, and get ABC quality developing done for a good price"
Yes it does take more time than just packing all your film in and out, so this must be factored into the trip, but it would certainly remove the possibility of Xray fogged film. If you bring a very small lightbox you can have entertainment for the flight home.
Just buying your film over seas and giving it to some untested local lab, even in a major city, can be disastrous.
I'll give you the benefit of the doubt - I assume you mean some non-pro lab / 1 hour place by "some untested local lab"? Otherwise, this statment is laughable. There are some very fine professional labs outside the US.