Keep the cat pipe to pass the MOT and use a decat pipe and enjoy a bit more ummph and better MPG, also block off the small Diameter pipe from the exhaust manifold that goes to the inlet pipe - this stops exhuast soot blocking up your intercooler and allows your engine to breathe clean air only.
I think the VOSA ruling in the early 1990's only applied to petrol cars as their is no diesel vehicle(uk spec) that has a cat until the latter part of the 1990's