Hmm. I saw on your profile that you're from the US, so you've got two options: either send an autograph request to someone in the UK and ask them to put some British stamps on a return envelope and send your request plus the stamped envelope to DT's fanmail address. Or, if you don't know anyone in the UK or don't want to do that, you can go to your local post office and ask them about international pre-stamped envelopes - I researched this a while back, and I think there are such things in most countries.
Which address exactly you'd have to send the letter to, I don't know. You'd have to either ask wihluta or google it. Fanmail.biz says this; I don't know if this is the right address, though.
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Hmm. I saw on your profile that you're from the US, so you've got two options: either send an autograph request to someone in the UK and ask them to put some British stamps on a return envelope and send your request plus the stamped envelope to DT's fanmail address. Or, if you don't know anyone in the UK or don't want to do that, you can go to your local post office and ask them about international pre-stamped envelopes - I researched this a while back, and I think there are such things in most countries.
Which address exactly you'd have to send the letter to, I don't know. You'd have to either ask
Good luck! :)