Deer: The Gambling Act of 2005 (here in the US) says that you can't charge for a non-skill contest if there isn't some kind of free back door entry method, otherwise it's considered a lottery. The question then becomes if the government considers this game a 'skill contest' or not because, if it is, they aren't required to have a free write-in method. Contest Law is insanely complicated so, they might just be covering their behinds.
I think using snail mail is just an easier way for the WGT to ensure they
aren't giving all the credits to one person with a few hundred email addresses. You'll have to work for your free credits a little. :D When you really think about it, it amounts to a pack of envelopes, a pack of index cards, a book of stamps and about 10 minutes of handwriting. You're out $10, but your're set to write in for as long as you need to build up a reserve to live on. All in all, I'd say that's a pretty good deal.
Mailing from the U.K. and using standard Royal Mail, I'd guess it would take about 2 weeks give or take. If you plan on writing in, you should drop them in the mail on the 1st or 2nd of each month to give them plenty of time to get there.