Unless you're in a private box with reserved parking (I was lucky enough to have both the last four times I attended) Boxing Day at Kempton Park can be a living Hell.
The poor man's Ascot is absolutely rammed and if you're funnelled into the car park in the middle of the course you're liable to be still waiting to get out at 6pm in the dark, well over two hours after racing.
I'm neither a traveller nor much of a racegoer, so I really can't be bothered to ever go over there to find out, but if you read people like Mike Parcej on X (who is English btw) you'll read tale after tale about the racegoer experience in Ireland being infinitely better on every level (quality, value for money, food and drink, accommodation, public transport, you name it) than in Britain.