Part of the criteria for opening a current account is that you promise to pay in a regular sum every month, probably a minimum of �250.
To open a current account, a credit search is carried out so each current account carries financial risk to the bank. They might not like having this risk multiplied too much by giving you 8 accounts. Especially if this means that you've got 8 overdrafts, 8 cheque guarantee cards (�100 limit?) and 8 chequebooks (say there's 25 cheques in each book, that's 8 X 25 X �100 that you can guarantee without actually having any money!).