I've rented a car when forced into doing this following a cancellation the night before. I ended up hiring a car and it took about 2h door-to-door, but that was when Hertz Gold Plus didn't involve paperwork on the day. It took about 2h from landing to check-in desk, albeit with pre-Brexit passport queues. I think this is doable but risky if there is significant delay, both at the airport or on the road. Depending on the advance notice, a taxi would cost the same as a new flight.