Please post this on the forum linked above. This forum is for completed trip reports. In the mean time here are a few thoughts:
The HP studio tour takes most of a day, including travel time. And you really need to be alert and rested before even starting out. This is not something I would recommend doing on your arrival day when you are likely to be jet lagged and tired, and possibly fuzzy headed.
Secondly, even with an early arrival time, you couldn't be sure of making your appointed start time at the studio unless you booked it for quite a few hours after your estimated arrival time. Your plane could be late arriving. Passport Control could be backed up. Trains might be running late. Etc, etc, etc. And it doesn't make logistical sense to get a hotel at Watford Jct and then have to move hotels again the next day.
I'd recommend going to your London hotel on arrival (preferably at or near the hotel you'll use for your tour). Drop your bags with them if it is before check in time. Then explore a bit of London on your own - preferably staying outdoors most of the day. Do the HP tour the next day when you'll be more rested. Watford Jct is easily reached by train from Euston stn. BTW, I hope you know that you need to buy your tickets months in advance. If you leave it til the last minute, your only choice may be a very expensive bus tour ticket.