If you check in to a hotel in Windsor, you can store your luggage there.
"The following items may not be brought into Windsor Castle:
· Bicycles, folding bikes, roller-skates or skateboards.
· Pets or animals (excluding service animals)
· Large bags that can not fit through airport-style security scanners"
Another option is booking a car service who will keep your luggage and the transport you to your next destination. (Airport, train station, hotel etc)
Scroll down for information:
Luggage Storage Per Vehicle
The first two I would contact for information would be:
What time do arrive at Heathrow? If you arrive on the Delta fight at 13:20, you might not have much time to visit the castle.
"March to October Open daily 09:45-17:15
(last admission 16:00)
November to February Open daily 09:45-16:15
(last admission 15:00)"Edited: 21 August 2017, 09:01