Going back some years now but I once returned from overseas owing someone forty pounds (in "our" money).
I thaught the best way to resolve this was to put four ten pound notes with a thankyou note in an envelope, seal it up safely and clearly address it and put on the reverse "sender" and my address, in small print. Job well done I thaught! Some days later it was returned to me by H.M.Customs with things to the effect "You're not allowed to send cash through the post to outside the U.K." and "We'll let you off this time" I was a bit disgusted but you can't argue with the law ! So, I ended up going down to the post office and transfering the money into the recipients account over there, and that worked. It cost me a bit extra 'though. This was a long time ago, I don't know how it stands these days, good luck.