[...] say in march or April the new models are out and you buy the cheapest mbp again. That's £2000 on 2 Mbps.
I suspect most people would resell the old one instead of keeping it, so if they sell it for £800 or so, then they've spend perhaps £200 additional to have a few months of ownership of the current model. Better yet, having today's current model gives a better ability to sit on the sidelines a while and allow for any early production issues to shake out. Maybe even wait until new systems ship with Lion. Might end up costing perhaps £300 differential, but if it buys six months that's decent value.
What will you be able to do with the next gen model that you can't do with the current one that justifies spending another £999?
Therein lies the key question -- I'd venture to guess that for many people the real-world differences won't be particularly noticeable.