I would probably start by asking your staff their nearest 2 underground stations. Upload the spreadsheet to something like ZeeMaps to pin all the locations to a

I would probably start by asking your staff their nearest 2 underground stations. Upload the spreadsheet to something like ZeeMaps to pin all the locations to a map and getting a visual idea of where clusters appear. You could even do this on your office wall. However is the best office one that has the shortest journey? Surely neighbourhood, space, cost & local facilities are equally important.