Start with the London station you can reach easily
The best-value rail route often starts with geography rather than the operator. Thameslink runs directly from St Pancras, Farringdon, City Thameslink, Blackfriars and London Bridge. Southern and Gatwick Express serve Victoria, with Southern also useful from Clapham Junction. A cheap ticket from the wrong side of London can lose its advantage after another Tube fare and a difficult cross-city change with luggage.