Long Layover in Heathrow

Well having access to the lounges makes a big difference. Download some books and videos. Occasionally go for a walk.

Coming from where and going to where? What time of day?

That 8.5 hours could shrink significantly depending on your travel details.

The biz lounge sounds like a good plan, especially if it has shower facilities.

Coming from Florence - going to Washington DC; where I'll have to catch another flight. I'm sure I will spend some time in the lounge - but I was thinking maybe I should have something to do other than sit around the lounge - only to board an 8 hr flight back to the States.

If you want to leave the airport, make sure to apply for the UK’s Electronic Travel Authorization.
https://www.gov.uk/eta

As long as you give yourself adequate time to get back to the airport, you should be fine. You will already be checked into your flight, but you will need to go back through security.

during the day, or overnight?

Beware that the traffic near Heathrow and the highways in the Metropolitan London area is terrible.
A trip from the airport into the city took us about an hour on the National Express bus.
The M-25 orbital is frequently one long parking lot.

You would need about 2-3 hours to do justice to Windsor Castle. A taxi would probably cost you something like $150 each way, at least.

I'm not familiar with Heathrow at all, but other airports that I've spent longer layovers in have had various and interesting art exhibits that I've enjoyed. Maybe check that out.
Happy travels!

Assuming it takes you one hour to get to the curb(or tube) of your terminal at Heathrow after immigration, etc. It often only took me 45 minutes, consistently, before the digital (?) exit, which is quicker but I haven't timed it. Also, assuming you want to be back at Heathrow three hours before your departure, you have 4.5 hours. If you took the Heathrow Express each way (quickest), you will use 30 minutes for both, plus the wait time. I haven't taken this in a long time so I don't recall how often it runs. So now you are down to 3-3.5 hours. If you stay in the area of Paddington, you could enjoy Hyde Park for some nice exercise before sitting on another flight. Grab a sandwich at the cafe in the park or a pub, etc. You would safely have 3 hours to enjoy your stroll, pub meal or stroll through the streets.

I think Windsor would be riskier as there aren't as many good back up transportation possibilities.

A cab from Heathrow to Windsor is more like 35 GBP, or at least it was in 2023.

What airlines are you flying?

Is this on one ticket? If no, will you have checked luggage? Would you be able to get the second boarding pass prior to your arrival at Heathrow?

If all of this is on one ticket, and you decide to leave the airport, there is no need to get back to the airport three hours ahead of time for your flight back to the US. If you do, you will have a plenty of time to kill in the lounge. (I fly out of Heathrow a few times a year. I fly business. I only have carry on bags and check in online. It has never taken me more than 30 minutes to go from entering the terminal to entering the lounge.)

You will already have your boarding passes and any checked luggage will be automatically transferred between flights. If you have carry on, do you want to store it at the airport or take it with you?

Also keep in mind that since you are in business class, you can use Fast Track security at Heathrow that has shorter lines.

If these two portions are on two separate tickets, do you have checked luggage? Will you be able to check in online prior to getting to Heathrow?

Thank you for all the replies!
OK - the flights are on one ticket - British Airways. I arrive at LHR at 8:30AM, depart at 5:00PM. I will check my bags (that, with any luck at all are filled with Olive Oil and wine!) I will have one very small bag with me that I can carry as a purse on my shoulder. The thought of leaving the airport, and not getting back in time for my flight does concern me. I am just hoping to make the most of my time.
thank you very much!

The Heathrow Express runs every 15 minutes. The M25 is irrelevant to National Express to London, or an any route to Windsor, as neither touches the M25.
A taxi to Windsor would probably go down the M4 but there are a couple of back road routes.
If for some reason it was looking dodgy to Windsor you could go to Runnymede (Magna Carta) instead via the Thames side town of Staines.

Since you should have your boarding pass for your US bound flight when you check in for your flight to London, and your bags will be checked through to the US when you check in at Florence, AND you get to use Fast Track security at Heathrow, there is no need to arrive back at the airport three hours ahead of time. Even arriving back two hours ahead will allow you to go through security and hang out in the lounge.

Regarding the lounges, there are three available to business class customers. Two, the North and South lounges, are located in the main terminal of T5 and are always crowded.

There is a third lounge located in Satellite B. (You get there by underground tram. Just follow the signs.) Most flights to North America leave from Satellite B or C. However, I can' suggest you head out there until you are sure you are departing from one of those. If you should need to get back to the main terminal, you will have to be escorted and finding someone to do that is not easy. This lounge is never that crowded.

Do check and see if you will need to transfer terminals at LHR. Often one will arrive at terminal three on inter European flights and depart from terminal 5 on US flights. This can all be done airside, but does take a small amount of time.