Best way to Heathrow?

Gloucester Road tube station is on the Piccadilly line. That will take you straight to Heathrow in around 45 minutes. So aim to be there soon after 9am, and just settle back for the journey. It''s too far away for engineering works to have been announced yet, at least a month too soon.
So check back in mid April for any engineering which is affecting the line that day. Sign up to TfL emails and final of any work will be issued on the Thursday, 2 days before travel.

Excellent, thank you!

Beware of journey planners that suggest circle line to Paddington and Heathrow Express from there. Yes it can be 10 minutes quicker theoretically but just over £20 more expensive if buying on the day.

But Heathrow Express tickets can be heavily discounted if bought ahead of time. We paid ₤7.50 each for our recent tickets. We bought them online about 6 weeks ahead of time.

Edit to add: pollyannapax, it sounds like you're on the Best of London tour? We just finished that tour, and had a wonderful time. Here's my trip report, in case you're interested: https://community.ricksteves.com/travel-forum/trip-reports/trip-report-best-of-london-february-19-25-2023

We always take the Piccadilly Line to the airport since we stay at hotels by the Gloucester tube stop. Since you’re leaving from T3, any train that says Heathrow will get you there. As already stated, the ride is about 45 minutes.

@ Jane's trip report is simply wonderful. Thanks for all the time you put into writing that.

HEX cheap tickets now have to be booked 5 months in advance- a recent change.

Thank you, isn31c.

Jane, I'm actually on the Best of England Tour. I haven't been to England since the summer of 1981, so I am very excited. Yes, I love to read trip reports, so will read yours as soon as I get off work.

Pollyannapax, We're thinking about that one; let us know how you like it.

My "home" tube station is Earls Court which is the next station from Gloucester Road. I've taken the Piccadilly line from the airport to there more times than I can count. From Gloucester Road to Heathrow, the trip will take anywhere from 40 minutes to 55 minutes depending on delays. (In January, I made it from Earls Court to Heathrow in 37 minutes including a two minute delay. Gloucester Road is only about three minutes further.)

It is a direct train with no changes. The fare is 5.60 GBP. Any other suggestions for public transportation cost more and will involve much walking and changes of trains.

A taxi will run somewhere between 75 and 100 GBP. A car service slightly less.

If you get to the Gloucester Road station by 9:30 AM, you should reach Heathrow by 10:30 without a problem.

Now, if engineering works are announced for that date, and there is disruption to the Piccadilly line somewhere along the route between Gloucester Road and Heathrow, this will change. They will offer bus replacement service on the closed portion but that will take forever. If that happens, get to Paddington and take the Elizabeth Line. Give yourself an extra 30 minutes just to be safe.

What date in May? If it's Coronation weekend, this may all have to be adjusted.

Thank you so much Frank II. No, it's not Coronation weekend, thank goodness.

I am no particular fan of King Charles III, but it is funny that I was there for his first wedding, and now I'll be there for his Coronation.

If you have a smartphone, download the TFL Go app. It will help you navigate the underground and buses as well as give status updates for the individual tube lines.

Done.

I will let you know Jane. I'm sure it will be fabulous