Is it your first half? Do you know that you are capable, when well, of easily completing a half marathon? Have you done the training? Without the cold, would you be able to complete it fairly easily?
I got a virus and then a cold in the last few weeks of training for the Abingdon Marathon last year - it affected my time, I'm sure, because I wasn't at my sharpest, but I did finish fairly easily. Everyone's different. Only you will know. I would imagine the Silverstone one is fairly easy to pull out of if need be - no need to finish if you're not up to it on the day. No shame in stopping either, if you're unwell. Having said that, there are lots and lots of halves, so you might just be better advised to pull out and go for another one.
Oh, and if it's on Sunday, you should by now be tapering anyway and doing minimal training... You won't get any better at this stage.
Edited: 27/02/2013 at 16:16