Since it is around the Washington D.C. area, there is a strong military presence, I think it was purchased as a tax free U.S. spec car back in the UK and once that serviceman had to return to the U.S. The military allowed them to keep the UK plates throughout the whole shipping process over there until they could regain U.S. Plates. It is similar to how a military personal who served in a base in Germany would get QQ transfer plates.
By driving through a secret tunnel under the Atlantic (but shhh it's a secret!)
But in all honesty, it might just be a returning US servicemember. It looks like the plate frame is sponsored by DriveMAS, which would make even more sense.