Insurers can use this to refuse a claim...

Not to repudiate their liability under the Road Traffic Act (i.e. damage or injury to a Third Party) they can't. They may pursue the policyholder to repay their outlay but they must meet the claim initially. There are very few grounds on which an insurer can refuse to meet a claim from a Third Party and the condition of the vehicle or its equipment is not one of them.

The last line of the advice the OP has cut/pasted is all you need to know; stick to what it says in the handbook and/or on the sticker on the door pillar, fuel flap or wherever the manufacturer puts it.

Both my cars, a 1.6/115PS Berlingo and a 1.2 Fabia have a tyre specification for well in excess of 100mph. I'm quite happy to stick with that even though neither is likely to exceed 90 and that only due to innatention in the 80+ limit on French Autoroute or an escape' manoeuvre caught in lane3+ on a UK m/way.

My car has a top speed of 140 mph - the original tyres were W-rated - I'm comfortable replacing them with V-rated tyres which would be adequate if I ever went to Germany and buried the loud pedal in the carpet on one of their unrestricted autobahns.

As long as the tyres design speed limit exceed the top design speed of the car, then in this should be fine in the eyes of the law. In practice, whether that design spec only holds true when they are new or when worn and old is another matter.

I'd personally very wary of specifying tyres that have a speed rating of 149mph against a car's top rated speed of 140, especially as some cars will go a bit quicker in middle age when they fully 'loosen up'.

My 15yo Mazda3 is rated to 115/116mph. The OEM tyres were v rated, but the handbook and door plate says Mazda recommend H or V rated tyres for 15in wheels and V rated for 16in ones for my model (TS2) and the TS below it. Not sure about the Sport model, which may have uprated brakes for the 2.0L model but perhaps not the 1.6 petrol.

Odd why they don't recommend H rated tyres for the 16in rims. When I changed down from the 16in V rated OEMs (I needed to change one alloy wheel and thought it was worth changing all four) to 15in, both the alloys and tyres were considerably cheaper (25-35%) and more so in H rated format (about another £5) each.

I went for the H rated (130mph) as that was enough over the top speed of the car, as I wouldn't contemplate doing near its top speed (or driving abroad in it) anyway.