Hi,
It's not possible to give you a ball park figure as insurance rates/premiums vary alot depending on many criteria. Check the fine print of the lease, it may require you to hold a UK license, so talk to the broker/dealer about this. If it's possible to lease on a US license here, then you'll need to look for insurance companies willing to insure you without a UK drivers licence. Premiums will be more expensive in this case and depending on the type of car you are leasing too - for instance will you be leasing an entry level car? a medium type and brand ? a prestige brand? an executive car? a sports car or a Super Car? your UK address/postcode, your profession and age will all make a difference.
How long is the lease? if more than a year, keep in mind you have to obtain your UK license after 12 months of being/living in the UK i.e, you can't use your US license for more than 12 months after arriving here. There's many topics about this in this section here.
Cheers, DtM! West London & Slough UK!