Ok, so it seems the "fronts must be same/rears must be same" is a bit of a fallacy.

ROAD TRAFFIC (VEHICLE STANDARDS) RULES 2002 - REG 42
42. Pneumatic tyres — carcass construction

(1)A vehicle with a GVM not over 4.5 t must not have pneumatic tyres of different carcass construction fitted to the same axle, but the tyres may have different cord materials and a different number of plies.

(2)However, subrule (1) does not apply to a tyre being used in an emergency as a temporary replacement for a tyre complying with the subrule.

AND

ROAD TRAFFIC (VEHICLE STANDARDS) RULES 2002 - REG 48
48 .Tyre tread

(1)A tyre on a motor vehicle must not have cleats or other gripping devices that could damage road surfaces.

(2)Except at tread wear indicators, a tyre fitted to the vehicle must have a tread pattern at least 1.5 mm deep in a band that runs continuously —

(a)across —

(i)for a vehicle with a GVM over 4.5 t — at least 75% of the tyre width that normally comes into contact with the road; or

(ii)for another vehicle — the tyre width that normally comes into contact with the road;
and
(b)around the whole circumference of the tyre.
(3)A vehicle must not be fitted with a tyre that has been treated by recutting or regrooving the tread rubber, unless the tyre was —

(a)constructed with an extra thickness of rubber designed for recutting or regrooving; and

(b)labelled to indicate the construction.

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