If you own assets in both countries, you will need a will in both countries. In that case, it won't be simple since you have to add special language so that neither will cancels the other will out.
If your net worth is in excess of the UK or US tax-free limit, you'll need someone who's clued in enough not to set up a trust in one country that will be subject to ridiculous tax rates in the other country. That's not cheap either.
The threasholds that apply are lower if you have a different domicle from your spouse. If you're not sure what your domiciles are for UK and US inheritance tax purposes, you may want to confirm what they are with a reputable dual US/UK accountant before beginning the will writing process.
If you own assets in just one country, and you are well below the taxable thresholds, you can probably squeak by with a cheapo will, perhaps even one of those will kits from Staples.