Return train ticket from Portsmouth Harbour to London Victoria.
Departing: Friday 3rd June 18:28. Return: Sunday 5th June 18:17 (others available)
Don't think there is any better train deal out there,
Simply add the stations and times in the ticket finder on the home page of southerntrains.com and press search. Once loaded, scroll down and on the left of the page you will find a box "find cheapest ticket" select return ticket you want and pay!!!
I have just got back from London and can also recommend Tune hotels, basic but for around £45 per night not bad and excellent location.
Hope this helps.