As others have stated, cash isn't actually used within prisons. However prisoners can use money, held on their account, to buy 'luxuries' (such as packets of biscuits or tobacco) and phone cards. Some prisons only permit prisoners to use money that they've earned while in prison, but most prisoners can also use money that was in their pocket when they arrived or is later sent to top up their account.
To the best of my knowledge, no prison allows visitors to top up a prisoner's account on visiting days. You need to send a postal order to the prison (made payable simply to 'The Governor'), stating clearly which prisoner the money is intended for.
Prisoners who've not been convicted can wear their own clothes in prison, as long as they've got a sufficient quantity to meet hygiene requirements. Some category C prisons and (to the best of my knowledge) all category D prisons permit convicted prisoners to wear their own clothes (subject to having a sufficient quantity - prisoners aren't allowed to have a mixture of their own clothing and prison clothes).
Prisons don't allow visitors to take clothing on visiting days. Clothing (together with other items, such as radios and MP3 players) must be posted to the prisoner.
Chris