If you wanted to get the most out of the lifespan you would have had to pay £425 anyway, now you can spend around £230 and get a newer slimmer model - or you can get a PS2 on it's last legs for £80 (or whatever they cost now), so really it's all swings and roundabouts.
Sony have a proven track record of actively supporting consoles for 10+ years, just look at how many PS2's are still being sold each month and 2010 has several new PS2 game releases. That is not to say a PS4 won't come out before the PS3's 10th Birthday, just that you don't have to worry about having the door slammed in your face like what happened to Xbox owners when the 360 launched (something MS have later acknowledged as a poor judgement on their part).
It sounds that you aren't an 'early adopter' interested in the bleeding edge of tech anyway so you could argue even if a PS4 came out next year (it won't btw) you will still be waiting 3 years to get one like you have done with the PS3...
I firmly believe that due to a number of factors like the cost of games development, the online firmware-upgradable infrastructure, the recession, and even the Wii showing graphics aren't the key to success, the lifecycles of consoles will be a lot longer than traditionally was the case.
When you look at where the PS3 is to where it started, and look at what has already been confirmed for 2010 like 3D firmware updates and motion controls it's hard to justify the need for a new console yet. Similarly with the 360 you've had the NXE and things like Twitter/Facebook and this year will have Natal etc. Traditionally all these enhancements would have been enough to justify a 'new generation' now they just evolve the current one in a very 'Apple' approach to feature upgrades. The only thing that can't really be upgraded on the current gen is the graphics - but many people don't even game in HD yet, nevermind 3D, and it hasn't stopped the Wii being a hit...
If the Motion Controllers for both consoles are a success I would say it could be as late as 2015 before we see a new console (the 360's 10th Birthday), if they don't make such a splash it could be as soon as 2013 - but I can't see it being earlier then this.
So even at the earliest (2013) added to your console purchasing habits (waiting and seeing for a few years) you could argue that at a minimum you could be getting 6 years out of a PS3, and at a £230 purchase that's a mere £38 a year - bargain!