http://www.legislation.gov.uk/uksi/2001/25/schedule/3/made

The Motor Vehicles (Approval) Regulations 2001

16 Tyres

6 Each wheel on each axle shall be equipped with a tyre which has a load capacity such that when the axle is loaded to its maximum permitted axle weight, the weight transmitted to the road surface by that tyre does not exceed that load capacity.

7 The speed capability of all tyres fitted shall be not less than the maximum design speed of the vehicle

Having dealt with them, I would try to avoid any situation which could potentially see an insurer refuse a claim or try to recover costs.

The same sort of question was asked years ago about Range Rover Classic's fitted with Mud Plugger tyres (Remoulds), I think the tyres were rated at about 80mph.

Ended up that as long as the vehicle was not exceeding the design speed of the tyres i.e. staying under 80 mph then it was fine, exceed it and then you would have a problem.

It's clearly a bit of a grey area regarding insurers - possibly something that will become more relevant with the increased popularity of winter tyres, as some manufacturers' winter recommendations have a lower speed rating than both the OEM tyres and the vehicle's top speed. Some cars a have a winter tyre setting speed limit alarm because of this.

As above, if you're bothered just check with your insurer.