Time for London sightseeing between planes at Heathrow

Play it by ear. If you land on time and weather's good, you're probably on track. Just before you leave, check the Underground's website for planned delays/disruptions. The less cabin baggage you have, the faster you'll get through security on your return. I doubt you'll have time for both museums though.

Hey, David,
I was just there (Heathrow) and I changed airports, different from you. The customs officer told me there would be no time to do anything but change airports in my 7.5 hour layover. I'd avoided the whole change airport situation for so long that I took her advice, which was geared mainly to what she described as horrible traffic on the ground between airports. This was at 9 am on a weekday. There were no problems traffic-wise outside of London proper and nothing shocking in the airport area. I understand that Heathrow is much better due to remodeling.
The lines all moved at a nice pace, even when crowded.

Knowing what I know now, and even though I'm a scaredy cat about being separated from luggage and missing flights, I'd do something, anything, in your situation, in London. Go have fun and you won't turn into a pumpkin.

Are you forgetting the maybe 90 minutes from wheels down to walking though the frosted doors on arrival?

1.5 hours arrival immigration
3 hours transport
2 hours check in
= = =
6.5 hours.

I don't see where you have the available time.

All depends on how risk averse you are, I suppose.

Terminal 5 security can be slow, but that is usually early morning.

You should be on for a bit of time if you are lucky with immigration on arrival, but don't cut your return too fine as if there is interruption to the Piccadilly Line you might get in trouble. The British Museum might be easiest as you can go direct to Russell Square. The National Gallery isn't too bad if you go to Embankment (by changing at Hammersmith). Don't think of doing both.

As you will already be checked in, your arrival time at T5 security must be no later than 35 minutes before your flight. Any later and you are refused entry. If you are taking off from B or C satellites you will need at least another 20 minutes.

Take Heathrow Express and you're in London in 15 minutes. That said, I wouldn't risk it. If reaching Cairo is your goal, why endanger that by making a quick trip into London? You would, more than likely, hate yourself for missing your flight. And, to be honest, trying to get into London will yield little more than a rushed, less-than-ideal wander through a museum instead of the slow savor you and the art deserves. Cooling your heels at Heathrow that long is a drag, no argument, but that will be forgotten the moment the plane lifts off for Cairo.

David,

as mentioned, i would play it by ear once you get there. Right now its all a lot of hand waving.

Since you know the how tos, its just a matter of "should i" once you land. You are also well aware of the consequences of doing what you want to do, so any screwups is on you, no one else.

also as mentioned, the Heathrow Express may be worth it, time wise, but thats something you will have to determine.

happy trails.

From Heathrow, take the tube (Piccadilly line) to Holborn. From there it's a 500 m. walk to the British Museum (less than 10 minutes). The ride takes about an hour and the train runs frequently during the day (maybe 10 minute wait?). Or for the National Gallery, get off at Leicester Square and then it's a 200 m. walk . . . if you don't take any wrong turns (a tad tricky . . have a good map and good sense of direction). I reckon it would take you at least a 1/2 hour to get from one museum to the other, so I don't think you'll have time for both.

Be sure to get your boarding pass for your second flight when you check in for the first one. It should be, but isn't always, automatic. Your checked bag should be checked through to Cairo, so you'll only have carry-on with you at LHR. There are left luggage facilities, but if you are changing terminals, it would be a waste of time to go to the departure terminal and store your stuff before heading into the city, which is why I suggest taking as little as possible. That may also speed up the security check when you return. The last time I went through LHR (indeed after spending some hours at the British Museum), I had the maximum carry-on allowed and not only was the security line slow (early evening), and they decided to take everything out of my wheelie, bit by bit (possibly because of the electronics in it - computer cables, camera, etc.). They then gave me a couple of empty bins and I had to repack everything before continuing on to my gate.

Since you are in the planning stage, can you give yourself an overnight in London or Heathrow? That would give you time to see the museums and get over some jet lag. Since you are many months out, you would also have time to look for reasonable rates for, say, the Sofitel, Heathrow.

Sometimes building additional down time into the schedule yields more flexibility and enjoyment rather than 7 hours in the airport. Just a thought. Another is exploring whether a trip to Windsor might fit into your 7 hour time frame.

Thanks everyone for their detailed comments and suggestions. Given the uncertainty of getting to and from London, I'll probably just cool my heels at the airport. The other option is flying Lufthansa to Frankfurt airport. That has a four hour layover to and 2 hour layover back from Cairo at Frankfurt. Thanks again. I'll post question about Frankfurt airport in the "Transportation" section.