Virgin Atlantic are not part of any airline grouping for lounge access and only accept their own Upper Class or Gold card holders.
For Pay On Entry, then go to the LGW web site and use the Airport Information tab to locate Lounges. BA operate from the North Terminal where there is an Aviance Executive Suite. From the web site, you can get a quote. With six of you, I suspect that it will not be worth it, ie £100. Having sensibly (IMHO) paid money for good seats, enjoy them and just find the best that you can in the Terminal.