Last night in Britain: Watford or Heathrow

Stick with the original plan.

Have the Watford hotel arrange a taxi to Heathrow.

Never fun rolling luggage anywhere for any length of time.

WB tour is nearly a day long adventure.

I guess it also depends what time your flight leaves and thus when you have to check in (for international flights at Heathrow check in is supposed to be three hours prior to departure) and whether you have enough time to make the journey by road Watford - Heathrow. And if you can find a taxi at an acceptable fee - it’s quite a way between Watford and Heathrow.

Personally, if your flight is before or around midday, I’m a fan of staying overnight near the airport. It takes the stress, or at least some of it, out of the equation! The only problem is that if you change hotels to Heathrow environs, you’ve an hour plus to get out there by tube and an hour plus to get back to get your Harry Potter bound train. You could, of course, get a hotel in the Kings Cross area which would give you close access to the Piccadilly line the next morning. Transfer from Euston to Kings Cross can be done by tube, although there may be quite a walk between platforms for the different lines.

It would also depend on which Watford hotel.

Which is it?

If, for instance, it were the Watford Junction Holiday Inn Express, that is adjacent to the Watford Junction station which is where both the official bus - the Night Bus - to the studio tour leaves from, and from the same bus station at the train station forecourt there is a RailAir bus to Heathrow.

So depends on the flight time, and when do you arrive from Scotland? Are you sure it is not an Avanti train from Edinburgh/Glasgow into London Euston? Is it a Lumo or LNER train into Kings Cross (platform 9¾?) ?

What time is the reservation for at the studio?

Great questions. We are indeed at the Holiday Inn Watford Junction. I am waiting for the advance sales, but we'll most likely be on the Avanti West Coast Line, which arrives at Euston at 1:10. Our tickets for the studio tour are at 4 p.m. I didn't know about the Knight Bus, that sounds intriguing as does the RailAir bus.
Our flight leaves just before noon.
Thanks again for everyone's experience, input and advice.

Question about your tour time at the HP Studios. If you aren't there until 4 pm, how long are you allowed to stay and tour it?

They are open until 10 p.m. that day. I'm not thinking we'll stay that late, but it should give us plenty of time.

you will go flying through Watford Junction station at 125 mph on the final run into Euston. There are food places in the station.

When you are ready, there are frequent (restricted timetable at the moment due to covid) trains to Watford Junction.

Don't bother with TfL Overground orange trains - they are the slow slow all-stops local train. Not all London Northwestern trains stop at Watford Junction but most do. Best is if it is first-stop, but even if it stops at Harrow and Wealdstone and Bushey on the way it won't take much longer.

There is a subway (underground passageway) under all the platforms and linking to the booking hall and the way out. Each platform has a small lift and large staircase down to the subway. As you leave the station the buses are to the left and the Holiday Inn Express is to the right.

You said that you are staying at the Watford Junction Holiday Inn. That's a different hotel to the Watford Junction Holiday Inn Express, and in the opposite direction but still very close to the station. I don't remember what it used to be. Turn left out of the station and just past the bus station you should see it in the right-hand side. The names are similar - be sure you know which one you are staying at.

https://www.avantiwestcoast.co.uk/where-we-go/station-information/watford-junction

Avanti west coast calls at Watford junction, so you don't have to transfer at Euston. Just get off at Watford junction.

Maybe the luggage pick up Service is useful?

https://airportr.com/en/ba/

the train from Glasgow comes down the main line, skipping the Midlands, and is right-away Euston from Warrington. To leave that train at Watford Junction would require a parachute and a guard with a sense of humour.

08:36 Glasgow Central [GLC] - London Euston [EUS] 13:08 4h 32m

Calling points - Arrives - Departs

Motherwell [MTH] 08:52

Carlisle [CAR] 09:48 - 09:49

Penrith (North Lakes) [PNR] 10:02 - 10:04

Lancaster [LAN] 10:38 - 10:38

Preston (Lancs) [PRE] 10:56 - 10:59

Wigan North Western [WGN] 11:10 - 11:11

Warrington Bank Quay [WBQ] 11:21 - 11:22

London Euston [EUS] 13:08