Six hour layover at Heathrow

I personally would find it very stressful to do this. Just getting through immigration can take quite a while. I've whizzed through in 15 minutes and other times I've stood in line for 90 minutes. Then you'll have to go back through security on your way back in - again could be quick or REALLY not quick. It's a bit of a crap shoot with the risk being a missed plane. I would opt for paying for a nice lounge, a massage, maybe a nap, and some good food. There's a fantastic whiskey shop in T3 so that may lessen the blow of not going into town?? :)
That said, the quickest way to central London is on the Heathrow Express...drops you at Paddington Station in 15 minutes if you want to risk it. Buy your tix ahead of time and it's not very expensive.

Perhaps buy a day pass to a lounge, shower, refresh, eat 'hydrate', and then sit in the flying sardine can.

When you figure out the cost of transport to/from and food, vs cost of pass it may not be a lot different.

I already have a nicely topped-up Oyster card so the cost on the Underground would be nil, but yes, the timing would have to work in my favour. I think it's only about 45 minutes to Covent Garden, so that in itself isn't too bad. Paddington is nowhere near where I'd want to be, and would cost more so the HEX is out if I decide to go for it.

Since I'm coming from the EU, don't I have to go through UK immigration anyway since they're not part of Shengen? Would it be possible to walk toward the "entering the UK" immigration queue, and if it looks like a horrendous wait just turn back around and stay in the terminal? Or are you committed once you pass a certain point? It's been 15 years since I last landed at Heathrow (T3 then, too).

I toyed with the idea of a lounge, and I think they're about £45 or thereabouts for 4 hours. That's certainly a solid "plan B" if leaving the airport is out of the question. Not sure I could justify a massage or spa visit, although it would certainly be a nice way to end the trip. I'll have been up since 0330 (London time equivalent) and a bit of a lie-down wouldn't be so bad! And yes, World Of Whisky will absolutely be the recipient of a fair chunk of my credit card balance. I've spent many a quid in that blessed shop, although it's a far cry from Royal Mile whiskies in Bloomsbury Street.

Perhaps I'll just day-drink their samples and lull myself into a stupor in the terminal. (I kid).

that could be a day of decision; you sound young like a younger traveler fleet on your feet so go for it; the only problem is the immigration border control folks are not going to let you in so quickly you will probably have to show them your ticket that you are leaving; and hopefully you will not take out cash for anything

you sound young like a younger traveler fleet on your feet so go for it;

If only! I'm mid-50s and hobble more than walk sometimes, but I'm a veteran of NYC and other large cities so I'm absolutely comfortable with London Transport and the London crowds. I spent a month in north London about 15 years ago and I do miss it terribly.

the only problem is the immigration border control folks are not going to let you in so quickly you will probably have to show them your ticket that you are leaving; and hopefully you will not take out cash for anything

I think I still have about £60 in cash left over from last time (and a FILM CANNISTER (!!) of coins). Of course by connecting boarding pass and ticketed itinerary will be on my person!

I guess the really big question mark is how long the queues typically are at immigration, and again at the security checkpoint when I get back. Here at Newark, invariably I get to the airport 2-2.5 hours before departure time and I end up sitting at the gate 10 minutes after I arrive, but since there's no such thing as TSA Pre at Heathrow, I know it will be a bit longer.

On the other hand, If I could somehow justify £100 for a 5-hour nap at the #1 lounge that, too, would be magnificent.

I already have a nicely topped-up Oyster card so the cost on the Underground would be nil,

Well I walked into that ambush fat dumb and happy. Add me to your trophy wall! :-)

You could walk to immigration, take a look, and if it seems long, keep following the signs to flight connections.

Look at it this way.....

One hour from LHR to Covent Garden. (45 minutes if the gods are on your side).
One hour back to Heathrow.
Be there two hours ahead for security just to make sure
That leaves two hours for immigration and time in London. Is all that hassle really worth it for such a short amount of time?

Personally, I would get the lounge pass. Free food and booze, plugs, tv's, comfy chairs. (Of course it depends on the lounge.) The only lounge I've been to in Terminal 3 is the AA lounge and it is very nice.

I would not find it so unpleasant to be at Heathrow for six hours as a layover, wouldn't bother me at all. The closest I did that was in Oct spending 5 hrs in Seattle airport as a layover after flying in from OAK, then flying on to London LHR in the 7-8 hr sardine can...very apt description since I book only Economy. If it's going to be multi-hour layover, say 5-6 hrs, I would pick SEA-TAC.

Isn't three hours the usual guideline for arrival at an airport before a transatlantic flight?

For flights to the States, the check in time is 3 hours minimum, so you need to be back at Heathrow at 11.30.

If your flight is on time, you will be through Heathrow sometime between 8.45 and 9.30, depending on the passport queues. Covent Garden is a good hour, allowing for public transport waits, so you aren’t going to have time for much before you have to head back. I wouldn’t want the stress with such a tight timeframe.

If you just want somewhere to stretch the legs etc and it is a decent day, consider taking a trip to Osterley Park and House. Gates open at 10am. Osterley is 5 stops away from T2/3.

If you really want to leave the airport why not consider going to Windsor, it’s a 15 minute taxi ride. It really all depends on if your plane arrives on time and how long it takes to go through immigration, but it might be better to stay at Heathrow.

If you are just changing planes at T3, not leaving the airport, you will not go through UK immigration. But you will go through a security check.

For flights to the States, the check in time is 3 hours minimum, so you need to be back at Heathrow at 11.30.
If your flight is on time, you will be through Heathrow sometime between 8.45 and 9.30, depending on the passport queues. Covent Garden is a good hour, allowing for public transport waits, so you aren’t going to have time for much before you have to head back. I wouldn’t want the stress with such a tight timeframe.

That lounge pass is beginning to sound increasingly nice. I'm not an Admiral's Club member, so it'll be a paid pass for me if I go that route. The 7-8 hour flight back to Kennedy is just the beginning of the final leg and "race to the mat." After I clear in NY, Airtrain to Jamaica, LIRR into Penn Station (you know what they say..."Lead us not into Penn Station...."), change there for a New Jersey Chance-it "train" and then a lift home by my better half. So that's going to be A DAY. From the NYC time perspective, I'll be waking up at 10pm, getting into Kennedy at 5:40pm, then easily another 2.5 to 3 hours back home.

As for security, they say 3 hours to cover their bases, but from a relative who lives there and travels to the US on business 3 or 4 times a month, he says it's never taken him more than about 45 minutes to an hour unless there's something Seriously Messed Up, and when there is, everybody knows about it well in advance.

You're right at that threshold where it's technically doable but how much enjoyment do you get out of it. I've never been in LHR immigration in the early morning longer than 30 or so minutes but as soon as you hit central London you'll probably have your internal clock on high alert thinking about what could go wrong on the journey back (or at least I would). In the past when I couldn't get a reasonable connection through LHR I've prolonged my layover until the next morning so I can have an evening in the city. But it sounds like you have long day and a decent lounge might be up your alley.

Yeah, this trip is on AA miles and that was the best connection I could get as a freebie. I could have chosen earlier departures on British Airways aircraft, but I'd have been paying about $350 in departure taxes rather than $91 on American metal. AA also had an earlier departure but there were no award tickets available for it when I booked.

I'm still leaning towards going into central London, and that's Plan-A unless the Tube's a mess or the queue at immigration is insane. I'm not going to any destination in particular, I just want to get a small taste of London again, even if it's just taking a quick stroll along Charing Cross Road or around Leicester Square, and having a proper cup of tea. I only really need two hours to do this. But it's easy to talk about here in front of a computer, not after being in hardcore walking/hobbling tourist mode for 12 straight days.

There are clear advantages to the lounges; that's still a definite possibility, and if I'm totally knackered or there are big delays on the Underground, etc, that's definitely a solid Plan-B. It might be another 10+ years before I can get back to London (I have far more interesting places I want to see), hence my desire to at least spend a tiny bit of time there.

I think I still have about £60 in cash left over from last time (and a FILM CANNISTER (!!) of coins).

Well that film cannister certainly dates it.

Sorry to rain on your parade, you may have another little delay. 15 year old money is probably no longer accepted. Old £5 and £10 notes have been replaced by plastic ones and the paper ones are no longer valid. You would need to change them at the Bank of England in The City. Some of the coins may still work but maybe not - depending.

I'd be prepared to use an ATM or credit card.

A 15 year old Oyster retains value forever but will probably have to be reactivated. That may take a little while.

Here's how the Bank of England will exchange your obsolete notes for you. Nothing easy about it but at least there's a way. With your LHR layover I'd advise against going into the City for the in-person exchange.

Your 1-pound coins ("round pounds") also won't be accepted anywhere. The Royal Mint (separate from the B of E) will exchange them but I don't know how. The new ones are multi-sided instead of round.

I have some old notes and coins that I plan to drop in charity boxes or wherever I can during my few hours in the UK next month. No point trying to do anything else with them.

Another reason to stay at the airport. As very hospitable people sometimes say in a completely different context, "Your money's no good here!"

Well, that's sad news indeed about the money. I'm not too worried about the coins, but the paper is a letdown. I just checked, and indeed all were withdrawn from circulation last month or last year. Well, if my side-trip is a go then a cashpoint will have to do, or better still, just use my cards as usual. Fine either way.

Like I said, this will all be a game-time decision. If we land late or things look truly embuggered then no. But I'm an optimist! And I do dread sitting in a terminal for that long, but it's better than missing the flight home.

Reasonable or insane? I opt for insane! Go to Winsor!

The Royal Mint doesn't have any exchange facility for the public for demonetised coins.