when ever i goto london i used to ditch the car at luton and jump on the thames link to kings cross then go where ever via tube, if i remmber right it was about £8 for the ticket and that covered us into london then the tube all over zone1 and 2 and return back to luton. there used to be quite a few streets in luton what you could park on free of charge. if not im sure parking in luton would me much cheaper than london.
it is a few year since we did it but im sure it wont be much different

Firstly, when did you check prices - with most companies the cheapest advance tickets normally only go on sale 12 weeks before travel (sometimes even later for weekends) rather than on a set date for the season like airlines.

Your first problem is that Fridays are the busiest days of the week so have the least number of cheap seats available. And just to make things worse, the line out of Euston is still being rebuilt at weekends (the Pyramids didn't take this long!!) so part of the return journey would be by bus. I suspect that because of this Virgin won't release many cheap seats because they are actually tring to deter people from travelling.

But all is not lost! If you can get to Wrexham then the new Wrexham and Shropshire Railway Company operate direct to London Marylebone and aren't digging the track up at weekends. They only seem to sell 8 weeks in advance but will do it for £48 return (half price kids 5-15 inclusive and further discounts if anyone hs a railcard). They also claim to offer big discounts for groups of 4 or more so may be worth phoning. If they do come up with a good price let us know because they've only been around a few weeks.

Liverpool to Wrexham is possible by train (Wirral Line and change at Chester) but the Saturday evening return service is a bit late and may not have a connection, it may be better to drive to Wrexham.