It's been a while, granted, so excuse me if I missed the point, but if your looking to hedge your GBP exposure to dollar strength, then I wouldn't worry too much about the trading / money management points raised (although relevant to trading, not here...)
some starting points:
* Calculate your USDGBP exposure, but bear in mind other pairs - if you are really keen, use a trade weighted index of Sterling strength, and hedge accordingly (much like the Dollar Index). For the sake of argument we will stick to USDGBP exposure, but the reality is that EURGBP (EURUSD by proxy) is something to ignore at your peril.
* Derive some measure of how much your position changes per 100 pips (this is why I mention the EUR point and GBP baskets. Relying solely on cable for this is leading yourself up the garden path) - I guess we are talking about Net worth, not just your Barlcaycard bill. After you are satisfied with the figure, simply take an opposing position in the spread bet market. If you do it correctly, you are securing your trade weighted index net worth - it will neither go up nor down.
* Be aware that you may have to fund your hedge position with additional margin.
* minimum Variance hedge ratio's are something to look into - if you can find a third party GBP index, you might get a better hedge by hedging this with two positions in EURUSD and Cable. It does involve regressions, the whole correlation/causation factors, awareness that correlations change, and so on...; but if it is a HEDGE that you really want, it's an improvement on simply shorting cable.