Hello, We will be in London for a couple of days and then flying to Rome from Gatwick. I have prebooked the Gatwick Express from Victoria Station leaving at 5:30am. We are staying at the Citadine's Trafalgar Square so I was thinking of just taking an Uber instead of the tube to Victoria station (looks like it only takes 15 mins to drive there). How early do we need to arrive at Victoria station to catch the Gatwick Express? We are doing mobile tickets.