I specifically asked the Lotus UK guys about a locking cap. They didn't and don't plan to offer one. And with the engine management system I was told it'd have to be of the exact same configuration as the factory one, in other words I couldn't just go to the parts store and try them until one that worked. This will trigger an engine management fault light.
I don't know about a locking gas cap, but someone could drill a hole in the fuel door cover and attach a lock.
Of course remove the door cover before atempting to drill the hole , and make sure the latch will latch so it has to be measured and all that stuff, maybe work with a mockup 1st to ensure the right placement etc.
Maybe someone will make an after market fuel cover with the lock already installed like the ones sold for the F150 and the H2. Actually the F150 one, might be the same dimensions.
I've been doing searches all over but I can't find dimensions.
Or we can find out who makes the fuel door cover and ask if they would offer a lockable version...
Edit: it would be ideal if it had also a stop inside so you know when it's locked.
When I was at Sports & Specialist in Princeton checking up on my soon to be delivered Elise (hopefully December delivery!!), the salesman and I talked about a locking gas cap and locking gas lid door....said he had ordered 20 of each. Not sure where he's getting them from (yet), but said they're 3rd party.