I imagine that has more to do with the prison itself and the procedure following the mail delivery and not the actual transit time to and from the UK. I might be wrong but it stands to reason that if you are paying the same amount of postage and sending the letter by the same method, the delay is more likely to be caused by the mail room or how backed up they are or whatever else is going on within the prison itself. Personal mail will never be top priority as legal mail is time sensitive so that would need to be screened and dispersed first.

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