We are traveling to London this spring, we have a toddler, we don't want to bring the car seat. Is this even possible with out a degree of insanity. We are staying at the intercontinental. We will be at both airports, heathrow and luton.
We went last summer with our kids (3 and 5 at the time). You shouldn't be in any cars, so you shouldn't need a car seat
Lots of options to get from LHR to the city (Heathrow Express, Heathrow Connect, or just the Underground). Looks like Luton is accessible via train and a short shuttle ride. For reference, we took the underground from LHR to the city (it was a lot cheaper than Heathrow Express) and took a train to LGW on our way out. For getting around London, we primarily walked, but used the underground or buses a few times as well (we got visitor oyster passes). We did use a taxi once and just held on to our kids (we didn't have local phones so no Uber).
You can reference: www.
gov.uk/child-car-seats-the-rules/when-a-child-can-travel-without-a-car-seat for the technical rules of car seats. Of course, there's a difference between whether you should and can ride in a taxi without a car seat, but we're not going to lug around a car seat all day/all trip cause we might use a taxi once or twice. Interestingly enough, sounds like this would make Uber illegal as they don't have a fixed partition. IMO you'll be glad you didn't bring a car seat - I think you'd go more insane with having to lug it around for a handful of uses.
P.S. Our kids had an absolute blast.