I have a long connection (6 hours) at Heathrow coming up, arriving at Terminal 1 and departing from Terminal 5, and can use my card in Terminals 1,2 and 3 but the lounges are obviously all airside.

I haven't been able to find a way to do it without having a valid boarding pass leaving from the relevant terminal, but is there any way I'd be able to reach any of those lounges? I don't pay much attention when arriving at airports, but I can't recall any way of staying airside on arrival without going through Flight Connections?

Thanks