In many years of traveling in Europe, almost all of which involved car rental, we have had three occasions to use the insurance, two with what came on the cc (once Chase and once Amex) and one with AMEX Premium rental car coverage. I have no complaints about the handling of any of them. If you go with what comes on your cc, be sure to talk to an agent to get clear about the coverage. Are tires and windows covered, for example? Do that if you go with the other insurer, too. As a matter of fact, do that if you decide to accept the rental company's insurance.
On our most recent trip, the rental company did put a hold on our cc because we didn't take their insurance; but it was 2000 euros, not the value of the car. That left us plenty of leeway to use the card. We always carry several cc's when we travel. so no problem anyway. This issue is also one you might be able to check on in advance.
One final thought - - 50 GBP, ($75?) is not a fortune, and is insignificant compared to the cost of your trip. If it reduces your stress to get the insurance from the rental company, maybe it's money well-spent. As for me, I'm a tightwad. I prefer the options I've outlined above. However, in my old age I am coming to appreciate things that reduce stress.