Dunno about the train but yr closest megaliths are the barrows on the medway valley near maidstone. Kits Coty, Coldrum etc.
Paddington to Swindon takes 1 hour by train, then you can get the 66 Stagecoach bus to Avebury - leaves every half hour. Apparently. Avebury's the site to see, there's so much and so many other things to see nearby.