If you're a non-UK resident, you will have to start from scratch. But if you're insured in France by an insurer that also exists in the UK, I would call them up and ask if they can transfer your NCB. Motor Insurance in the UK is expensive in general. Without knowing what your vehicles are exactly, I'd say anything less than a grand is the norm. You are also seen as being a greater risk over here because you've spent the last 15 years driving on the opposite side of the road.