From TfL-
Due to planned engineering work on Saturday 20 and Sunday 21 January, please allow at least 90 minutes for your journey to Gatwick Airport from Victoria or London Bridge. Rail services will run on a diverted route from Victoria, departing every 30 minutes. Alternatively, trains will run between London Bridge and East Grinstead to connect with a replacement bus service to the airport. This engineering work will also affect trains to Brighton and the Sussex Coast. This is due to Network Rail works.
Heathrow- On Sunday 21 January there will be a reduced Elizabeth Line service from Terminal 4 due to HS2 work at Old Oak Common, and no Paddington to Liverpool Street and Abbey Wood trains cross London trains. Shenfield line trains will run to and from Liverpool Street National Rail station (not the Elizabeth Line station). Heathrow Express are also running a reduced half hourly service.