When I went to the UK, I realized the airport was about an hour away from the city by train, which is much cheaper to take than a cab all the way there. I took the train to the Picadilly stop, which is in the heart of the city and is very touristy and bustling and busy, and Chinatown is walkable from there. If you wanted to spend an hour on the train both ways, you could potentially do that? I'm assuming you'd want to get back to the airport in time to get to your next gate (the airport is huge and it takes forever to get from one place to the next, especially if you don't know it, and there is a shuttle to get from one terminal to the next which is not walkable). I would give myself an hour before my flight to just be safe.
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When I went to the UK, I realized the airport was about an hour away from the city by train, which is much cheaper to take than a cab all the way there. I took the train to the Picadilly stop, which is in the heart of the city and is very touristy and bustling and busy, and Chinatown is walkable from there. If you wanted to spend an hour on the train both ways, you could potentially do that? I'm assuming you'd want to get back to the airport in time to get to your next gate (the airport is huge and it takes forever to get from one place to the next, especially if you don't know it, and there is a shuttle to get from one terminal to the next which is not walkable). I would give myself an hour before my flight to just be safe.
So, maybe you'd have 2-3 hours in the city?