I leased a car last month, and didn't run into the insurance before leasing thing. But I also came there in my own car, so they probably just (rightfully) assumed that if I had my own car, I had insurance.
I was able to insure with Geico, and the price I'm paying with them is incredibly cheap, even for experienced UK drivers. The only insurance company that was at all close (and still hundreds higher per year) was Aviva. They're not on any of the comparison sites, so ring them (don't use the website) and see what you get from them.
If you're only here for two years, maybe get car insurance as part of your deal with your employer, if possible?
Also, I echo strongly the prepare well in advance to get your UK licence. The test is definitely nothing at all like American tests - much more strenuous and strict. After I passed mine, I was told that it was considered to be the most difficult driving test in the world. Not sure it is that, but it is definitely up there. Allow six months to get everything done - provisional, theory, lessons, practical exam.
I have an amazing instructor who has taught a few Americans and drove a truck in Germany - he's used to switching sides of the car and road - but he's in Manchester and I'm not sure he'd go to Chester, but he might. Let me know if you'd like his info!