Iam64 and others from wikipedia - you ned a permit number as well as the word Freepost.
"Freepost is a postal service provided by various postal administrations, whereby a person sends mail without affixing postage, and the recipient pays the postage when collecting the mail. Freepost differs from self-addressed stamped envelopes, courtesy reply mail, and metered reply mail in that the recipient of the freepost pays only for those items that are actually received, rather than for all that are distributed.
In the UK, freepost envelopes can be hand addressed by the sender; this simply requires including the word ‘FREEPOST’ and the recipient’s permit number as part of the address. (This technique can not be used to send mail to merely anyone, who would then have to pay for it; the permit number and the recipient address must match for the recipient to be billed .) "