London station to Brighton - cheapest train ticket

sarahsilb

I want a single train ticket to go from a London station to Brighton tomorrow.
Cheapest single ticket with a Railcard is £21.45. Day return from London to Brighton is £21.50 BUT if I get a day return from Brighton to London it's £12.50!
Is there anything to stop me buying the cheaper return but just use the London to Brighton leg (at 8ish in the morning)? And would I be able to buy online & collect from London ticket machine?
Thanks in advance

JGB1955

A single is £8.95 on Split My Fare | Cheapest Train Ticket Search  Leave London Bridge at 8:05

James_Londoner

A Super Off Peak Return to Brighton from any Thameslink station in London is £9 with a railcard.

Travel from St Pancras / Blackfriars / London Bridge etc. 

sarahsilb

Thanks both of you, now sorted!