Just noticed this in a Travelzoo email...
Megatrain has just increased its services by almost double from London to Portsmouth and Southampton, so there are loads of return fares available for £2.50, including the 50p booking fee.
Tickets are available to book now for travel from 14 October until 14 November. Look for the £1 fares each way.
How to book: Click on the link and select your routes and 'Train' in the 'Travelling by' drop-down.