Business Link is good for general business advice but you're obviously better off with people that understand your technology, business model, etc. Again, general business advice is one thing but for UK specific tax and legals you need someone over here to talk to.
Open Coffee (http://opencoffee.ning.com/) is a great place to start, there's probably one near you. We've got roughly 3:2 - start-up businesses to lawyers, bankers, accountants, etc at OpenCoffee Bristol.
LinkedIn is OK, but better for finding potential mentors, advisers, etc, rather than general Q&A. I've found it a bit spammy with US 'experts' plugging their websites.
Twitter is rapidly becoming a real time Google and there are plenty of good UK people that will offer advice. Hat tip to http://twitter.com/z303 for pointing this thread out to me. :)
Another avenue is to get friendly with your local University incubator. Even if you're not a graduate, they'll usually dispense pretty good tech/start up advice and (should) be good hubs for finding additional advice.
As a last resort there's always email! If you talk to someone at a networking event, drop them a note to see if they'll be willing to chat. In this climate most people are willing to put in a little business development good will. Obviously they'll want payment for any heavy lifting but you should be able to work out what you need far more accurately before making that decision.