The Royal Mint have been wrongly accused by accyman, they don't print banknotes. The Bank of England are responsible for printing our paper currency.
From their website:
The Bank of England will always exchange its old notes. Notes may be presented for payment either in person or sent by post (at the sender's risk) to: Dept NEX, Bank of England, Threadneedle Street, London EC2R 8AH.
If you want more info:
Bank of England | Banknotes | Withdrawal of the Elgar £20 Note - 30 June 2010
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