Again, there is no RCC departure lounge at LHR when you're flying on to Europe! (If there is an RCC, it's only at the gates to the US.) There's some other Star Alliance lounge, but it's definitely not called an RCC!

The only lounge you might possibly qualify for is if the airline you're arriving on in business class on has an arrivals lounge, and your business class upgrade ticket qualifies you for that. In that case, you'd have to leave the lounge plenty of time ahead of your connection, because this lounge would landside at your arrival terminal, so it's after you left the lounge that you'd have to change terminals, go through security, etc.