We used Toyota insurance when we first arrived back in the UK as we bought a 2nd hand car from a Toyota dealer and they offered a discount. Subsequently I have used the online comparison sites to find insurance. We both had UK licenses.
You may experience some pushback from companies when they find out your wife has a US license but, although it might cost a little more, it is doable.
Be prepared for the cost of insurance to rise when she gets her UK license, as she will be treated as a new driver.