Originally Posted by
sdix
You could take an Uber or Bolt to Windsor. It's about a 25 minute drive or go into Central London via tube or train (about the same time). As mentioned above you are at the hands of TFL going into London. Any number of disruptions could derail your plans.
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I would see how you feel when you land and make a decision when you arrive. You might be early with more time to kill than anticipated but I would have thought an 8 year old on their first international trip might enjoy seeing the outside of Windsor Castle so this feels like quite a good option to me.