You have access to three oneworld lounges - the Business Class section at Galleries South at T5A (up two sets of escalators - the first level houses the First Lounge and CCR), Galleries North at T5A and the T5B lounge.
All three offer identical facilities but crucially the one at T5B will be considerably quieter.
There is, however, a big but in all this - and that's the high likelihood that your flight to AMS will leave from T5A. You can get back from T5B to A, but you need to know what you're doing - you have to walk back through the passageways at Level -4. Essentially you walk against the natural flow in the departures tunnel.
Any other method (i.e. the shuttle) will land you at the border, at which point essentially your booking is in tatters because you can't pass security a second time on your boarding pass. The reset process is laborious, it isnt always easy to find someone to help, you may lose seat assignments in the process, and there's a decent chance you'll end up missing your flight.
Fortunately, this being FT, there's a guide in this forum on how to use the passageway. Unfortunately some of the photos are no longer hosted, but the description works. You'll also find the "Staff only" notices in the lifts have long disappeared, so you no longer feel like a naughty schoolchild accessing them!
https://www.flyertalk.com/forum/brit...assengers.html