Most of the big companies like No Limits will rent you a bike, but it's worth comparing costs with a company such as the one linked to by Griff-SV.
If you fancy doing one with some people from the forum, we're doing
Brands GP on August 12th.
I'm of a divided opinion on renting a track bike. For a few outings a year it's definitely cheaper than running your own track bike, especially as you don't need to transport it. But I think it's rather nice to use your own bike. It's just tarmac, and the chances of being knocked off by someone else are very slim, almost every off I've ever seen has been an unforced error by the rider. This is good news as it's in your control to ride the way you want to.
Jambo