Britain no longer has the ability to produce shells for the Challenger's gun. They don't use the NATO standard barrel either, so basically when they're out, they're out.
Edit: the source for my claim. The original link is dead, so I guess we'll just have to take the wiki article's word for it.
Britain in a fucking nutshell. Weight is kilos unless it's people, in which case it's stone or pounds and ounces for babies; fluids are ml unless it's milk (excluding plant milk) or you're ordering it from a pub, in which case it's in denominations of pints: distance is taught in km but all roadsigns are in miles, etc.