I dont think that you are likely to be made homeless, you just might have less choice available to you, you sound in a reasonably good position, and at the end of the day, although lenders are being more cautious, they have to lend otherwise they dont make any money!
I think that your worse case scenario is that you have to remain on a standard variable with Northern Rock, not that bad really, considering that interest rates are the lowest that they have been.
i wouldn't necessarily go and get a valuation - lenders are likely to want to use their own anyway. Just be aware that the value may have dropped rather than increased, so that is a mortgage stresses a minimum ltv of 85%, you may not be eligible.
There will be people who took out mortgages of 110% on properties that have dropped in value - they are the ones that will be most concerned.