You have no choice in this. Your main line arrival station in London will depend on where you are travelling from.
It has its own station - Kensington Olympia.
All depends which station in Kensington - go from Harlow Town to Liverpool Street on the train, then take the Circle Line (in either direction although eastbound's probably a bit quicker) to South Kensington station.
London St Pancras International Station.
Paddington
St.Pancras
London Paddington.
the London train station
The busiest train station in the UK is London Waterloo.
train from guildford station to London Waterloo
Take a train from London Waterloo station.
London Paddington. Catch a train heading for Bristol Temple Meads.
Kings Cross Station