PlusNet's > click here < premier package allows you to request multiple IP's.
It's £21.99 for up to 2mb, from April up to 8mb.
One of my colleagues runs an exchange and web server, using his plus net premier account, each on different IP's.
P.S. Being cheeky, if you do go with plus net, please put me as your referrer. my ID is RCCLTD. Thanks.