At 10 people, it's already cheaper than other services, which usually charge a few bucks per user.
I love this idea, though. I just feel that your approach of giving a full VPS is a bad idea. People who want a full VPS will buy a full VPS for much cheaper than the 20GBP you're offering it for.
What you need to do is multi-tenant your hosts, and isolate them from each other. Then you'll get costs savings that you can pass on to your customer; which will put you in line competitively with the likes of HipChat and Campfire.
Great idea, can't wait to see what comes of it!
This is bad. You're throwing resources away and increasing your management overhead. It would make more sense to have some sort of bouncer-type proxy to determine the source then route through to a dedicated ircd instance on a shared box (e.g. x ircd instances, one per customer).