My wife and I are travelling on frequent flyer miles (class X) on UA and TG from EWR to BKK via LHR, and return via FRA. 4 and 5 hour layovers in LHR and FRA as well as a long return wait at BKK. At present we do not even use a Mileage+ credit card, rather Chase Sapphire. I do have a couple of United Club passes (just 2); however, there are no such clubs in any of our in transit terminals.

Mostly, we just want a lounge to find a quiet, comfortable place to sit and get a cup of tea.

Any suggestions for any airline lounge access at LHR, BKK, FRA ?

Can one purchase one time use passes or temporary memberships ?

Used to be a card and or membership subscription that granted access to almost any club. Still around ?

Thanks for any advice.

Priority Pass is probably your best bet. Can possibly purchase one-day passes to Servisair lounges in LHR.

You have no access on the basis of your ticket class (award economy).
And your two UC passes are no good.
At FRA, you can pay to use the Luxx Lounge.
It's landslide ... take a look here
At LHR, you'll be going from T4 (UA) > T3 (TG).
There are a couple of options here

I think all the third-party lounges accept pay-per-use one time purchase for entry.

If you arrive on time in FRA take the train downtown, hang around for a couple of hours and take the 15 min train back to the airport.
There are also 2 hr walking tours of Frankfurt; they pick you up at the airport, take the train, give you a tour and walk you to the train station.
You get out of the airport, get some sunlight and Frankfurt is actually nice. Not Paris nice but small town feel nice.

By the way, Frankfurt airport is not bad; lots of place to have some food, drinks, coffee or some shopping.

I'd head the other direction, to Mainz or Weisbaden, but same idea. Get out and see a bit of town.

Priority Pass can work but the options aren't great. At BKK you get in to the Louis Tavern Lounges (there are many of them) which really aren't anything special. A quiet booth in a restaurant would be just as comfortable, have better food and probably less expensive for two guests. At FRA it is the LUXX Lounge or the Sky Lounge. And at LHR T3 it would be the Servisair lounge or the No.1 Lounge. Again, for $30 each or so I think you'd do better in the terminal, but I have a somewhat skewed view of most lounges lately, I suppose.

If you are just trying to get into a lounge this one time, just go straight to a third party lounge (not an airline elite lounge) and buy a one time admission. Pretty expensive though in my opinion. Usually $30 - $45, per person.