I've been to many of the locations in the game, both before and after playing it. I was at university in Edinburgh, so obviously I've been all over that bit. It certainly makes navigating the city in-game easy for me because I know it IRL anyway!
I've been to Derwentwater and the Derwent Reservoir, Ambleside, the White Horse, Stonehenge (Standing stones) and Glenfinnan Viaduct in the past.
However, I have revisited Edinburgh since getting Horizon 4, and once again climbed Arthur's Seat to get an appreciation of the accuracy of the game (to which I was pleased to see was very good). It was fun to compare roads in the city to get a feel of what roads were changed or widened for the purposes of the game.
Since playing the game, I have visited: Bamburgh Castle and beach, Broadway in Worcestershire, stumbled across Roseberry Topping near Middlesbrough (The Great Ridge in FH4) and driven through the Yorkshire Moors (Astmoor) to experience that part of the country.
Really, Tarn Hows is the only major identifiable landmark on the map I haven't visited.
So yeah, I'd say I was a pretty good person to answer this survey for you