I'm an American working in the UK for the past 10 years.
Because of a tax agreement, if you pay your UK taxes, you will not owe US taxes for that period.
False! The US is unusual among nations in that you do have to pay taxes to your home country even on money you earn elsewhere. Fortunately there is a lower limit, not sure what it is this year, somewhere around $80,000 I think, subtract that from your UK salary and that's what you pay taxes on. You must file in any case.
I recommend looking into the Highly Skilled Workers visa - this is a points-based visa, which isn't tied to an employer. They appear to have changed this recently, I had the predecessor to this, HSMP. If you get an employer to sponsor you for a work permit, you are only allowed to work for that company, so changing jobs is awkward. It's also difficult to find an employer to sponsor you first time around.
insurance will be a minimum of £3-400 for a man with a clean record
Try £1,000 as someone with no record - you are a new driver, even if you've been driving for 20 years in the US. I'd recommend not bothering with a car if you're going to live and work in London, much less hassle and bureaucracy.
Yes, the cost of living is higher, especially in London. Also, you have no credit history. Salaries are stagnating, but there are still jobs out there.
You don't need health insurance, if you're working you'll be covered by the NHS. Whatever you've heard from American politicians, and even from bitching Brits, the NHS will do you fine. You can get private insurance, particularly through an employer, but I've found it to be a waste of money. Many will disagree ...
Right now, we're suffering the credit crunch the US gave us
*cough* *cough*