The British Motor Museum in Gaydon is great. I wouldn't expect to find Lotus products there but everything else is interesting.
Land Rover, Leyland, MG speed cars, Jaguar and Healey all have cars there. I think you can do the Land Rover off road there now.
https://www.britishmotormuseum.co.uk/The best place to see English cars in England is at the Goodwood Revival. Even the outside the event pre-70's parking lot is as impressive as any museum. See if you can time your trip to line up with Goodwood. It is always best to see these cars come to life and chat with the owners.
Haynes Museum is ok, as it the Coventry Transportation Museum. I wouldn't go out of my way for these. For non car stuff the Churchill underground is impressive as is Bletchley House (code breakers).
We had fun at the Morgon Factory. We were there for several hours on a self guided free for all. Didn't see anyone working just chatting and having tea. It is no wonder the cars take years to produce and cost a lot.
If you insist on seeing only Lotus cars the Barber Museum here in the US is pretty impressive, but you will be distracted by the millions of motorcycles that you didn't know existed and now can't live without.
Hope this helps.
Dean - Moved to Placerville 7 years ago... and never changed by profile;)