Just priced the MPS5`s via Blackcircles and in the staggered setup the OP would save £75 in total if he went with the same size all round , not worth any grief from the insurance company should it go wrong.
If it was a cost issue then they could look at a lesser brand (Hankook / Goodyear F1) which should save them money and keep the insurance and the cars dynamics happy.
K