I wouldn't import it. Getting it reconfigured to meet federal and state regulations, while doable, will cost you money and a great deal of agrevation.
However, if you are only going to have the car here for one year or less I believe you can bring it in as-is keeping your MOT registration. You will have to certify and subsequently prove the vehicle was exported by the of that year. This is all from vague memory but it might be your compromise solution. Who knows, you may not like living here.
2015 BMW M5 Sedan M-DCT w/Competition Pkg and Carbon Ceramic Brakes, Driver's Assistance, and Executive Pkgs, B&O, Alpine White / Sakhir Orange and Black Full Leather, lots of carbon fiber, and a Trunk Monkey called "Der Stig".