Heathrow layover

You'll need to return 3 hours ahead of your departure which is 2:40. Take the Heathrow Connect into London Paddington and walk around London. Find a pub and have lunch. Meander since you have that opportunity. If you have the budget, you can take the faster Heathrow Express (same platform) that departs every 15 minutes and takes 15 minutes to get into Paddington. The Heathrow Connect takes about 25 minutes, is much cheaper, but leaves every 30 minutes so if the Connect isn't the next one, take the Heathrow Express.

If she is arriving off a transatlantic flight, it maybe 11am before she is out of the airport, so about 12-12.30 in central London. Flying into Europe, she will need to be back at Heathrow at 3.40, so leave London at c 2.30 to get back to the airport. That’s about 2 hours, assuming her flight arrives on time.

Will she have luggage? She will need time to put her luggage in storage which will eat into her 2 hours.

If arriving from Europe, she will need to be back at Heathrow at 2.40.

Instead, I would suggest that she takes a taxi and heads to Windsor.

Where is she coming from and where is she going?

Jennifer has the schedule nailed. I for one would not go anywhere, but I am the king of long layovers (short ones make me very nervous) and I plan to stay at the airport and make the best of it. That means doing all I can to get a lounge pass, researching restaurants, etc. Yes, she could go to Windsor, it’s pretty close, but she’d likely be very tired from the (presumably) overnight flight, and maybe just resting, eating, napping etc. is in order.

It's unclear whether the traveler will be going to or from Europe, but from the 9 AM arrival time, I am guessing she will be just off an overnight transatlantic flight. I'm non-functional on arrival day and would explore the possibility of a room at the relatively inexpensive Premier Inn near T4. I don't know that they do day rooms, but I think they'd be happy to charge her the overnight rate for the period she needs. The current rate for October 3 is £74. (It can be half that if you book a non-cancelable room well in advance.) This is a newish hotel, quite large, and I don't think it's able to fill its rooms on most nights (thus the incredibly low rates for an airport location). I believe there's a good chance she could just show up at the hotel and get a room, but I can't guarantee they'll have a clean room for her at 10 AM if she doesn't book in advance, and I don't know how the walk-up rate compares to the Internet rate.

She could tentatively plan a trip to Windsor and make the call once she's on the ground in London, checking with the Premier Inn at that point if she's not up to heading to Windsor.

Interesting how 8 1/2 hours shrinks in the face of advance check ins and RT transport to relatively nearby destinations......
Upon arrival check the weather and consider making the hop, skip and a dash into Windsor for the grounds and building interiors offer a refreshing pleasant walk. If you are faced with drenching downpours I would suggest exploring the confines of the airport and staying dry (not a good thing to be "moist" and trapped in an airline seat).

(not a good thing to be "moist" and trapped in an airline seat).

Well, the air is so dry in the plane, your moisture problem soon evaporates.

If she is arriving from North America, she may be tired from lack of sleep. She could rent a room at the Yotel in terminal 4. They rent rooms by the hour and you can arrive at any time (book in advance).

I forgot the Yotel! For just a few hours, I think it would be cheaper than the Premier Inn, though it's probably a lot likelier to have no space available if she tries to get a slot at the last minute.

Many thanks to all for your help with this. She's weighing her options... - JLS

Ok, here it is Oct 3. Have you heard from Sis regarding her layover at LHR? Such as how she decided to use the time, how it went?

Yes, Tim. Sandy (aka "The Whirling Girl") is admirably adventurous and traveled to central London by way of the Heathrow Express train and the Underground Victoria Circle Line. She reports that she got a good--if fairly quick--look at Parliament Sq., Westminster Abbey, Victoria Tower and gardens, and more. She says it was fairly easy, not too crowded, fun, and that she was back at the airport in plenty of time without ever feeling harried. I hope she's now getting some rest on the last leg of her eventful 28 hour trip to Istanbul. Thanks again from both of us for everyone's advice and interest. JLS