I was in the same position about 18 months ago, I could see where the next-gen was heading, and I didn't like it, so I decided to step out early.
I looked at it this way, over a 5 year period.
Say a new console is £300 + 2 launch games at £40 each, and an extra controller £30, then a subscription to PS+ or XBL £35, then maybe 3 more games in that launch year, at £40 each. Then each subsequent year you buy maybe 2 games brand new at £40, then 3 more at cheaper prices/second hand at about £15 each, and a renewed sub for £25.
Over 5 years, that makes £1165.
This PC I just threw together for you
http://uk.pcpartpicker.com/user/metalshade/saved/1ExJ , costs £815 (Just tower and OS mind you.)
Now, buying a PC will change your gaming habits, you will have a HUGE extremely cheap (I mean, 6 games for £7.50 bundles) backlog, and will be much less inclined to buy brand new, and if you do, it costs £29.99 instead of £39.99.
But if you went on STEAM with the £350 difference, you could have a library of about 250 games, including some brand new releases mixed in.
Edit: Heck after 18 months of owning a PC my Library is a measly 157 games, but I bought maybe 2 brand new, and never paid more than 22.50 for anything. Got Borderlands 2, with all bonuses, preorder stuff and the season pass all for £22.00 on day 1.