The fastest way to send a written message to a prisoner is via the ‘email a prisoner’ service. There is a small charge per message but it is still cheaper than a stamp and usually faster than a letter. See www.emailaprisoner.com
Prisoners can also send and receive post. The full postal address of the relevant prison can be found at www.justice.gov.uk/contacts/prison-finder.
Please put the prisoners name and prison number on, as well as the senders details.