"It's why you drive a sensible car in the first place....." Nope, I drive a "sensible" car most of the time because I can't afford to drive my weekend toy all the time. It also doesn't have a roof so not much good in the winter! But it is a lot more fun than my "sensible" car, even at legal speeds.
".....especially in cars that are clearly too powerful for the average British road....." A complete fallacy. No car has a constant power outptut, it is the driver that controls the power with the accellerator. A Ferrari is just as well suited to the average British road as a VW Polo mentioned above, it is the driver that is the variable. It's exactly the same as that old favourite of the safety brigade - "speed kills!" - when the reality is it is not speed per se but INAPPROPRIATE speed that kills. You can do stupid speeds in just about any car available to the public (I remember a car review for the old Fiesta several years ago that asked why do even the lowliest hatchbacks need to be capable of anything more than 70mph?). Speed alone does not kill otherwise racing drives would all have a one-race career.
Inappropriate speed is a driver issue, and it is effectively a time compression issue. If you are going fast, you will travel a certain distance in a shorter time - simple physics. If that distance is the one you have before crashing then you have more time to react if you are going slower than if you are going fast. Inappropraite speed reduces the time you have to react to the unexpected, it would be better to say it is the lack of time to deal with the unexpected that kills you. If you are travelling down an urban road at the legal speed and a pedestrian steps out in front of you, you will have more time to stop or swerve round them than if you are speeding, whether you are in a Ferrari or a VW Polo (the Ferrari will actually stop better). Same goes for racing down country roads - you should always go into a corner thinking "what will I do if there is something in the roadway ahead?". If you are going too fast to avoid the accident then you are going too fast, regardless of the car you are driving. But, if you are on a completely open straight, with clear visibility for miles, then 100mph is just as safe as 60mph. If you have a controlled environment like a track it is even better and safer. And a track is recommended for finding out exactly how your car behaves on the limits, as you really don't want to be finding out for the first time in the example above with the pedestrian.
As you gain experience you get better at predicting problems. Bikers, for example, learn to look for diesel spills at the exits of petrol stations as that is when they are accellerating away, and where trucks often spill fuel from over-filled tanks (this is even worse in the rain as diesel stains will seep to the surface of tarmac in the rain, making for a very slippery combination of water and oil). Experience will help you judge the appropraite speed (and this includes knowing when it is actually better to slow down to LESS than the speed limit), an advanced driving course will also help. But the best advice is to assume every other road use is a complete twonk and will do the worst possible thing at the worst possible time (a given for Volvo drivers, it seems!).
And as for roadtests, my weekend toy gets serviced by a shop up near a proper circuit and also take it for a spin round the track, to make sure it is behaving as it should at high speeds. I wouldn't give it to an oridnary garage to test as I wouldn't expsct them to test it the same way. Maybe the Ferrari owner from Eastbourne assumed the mechanic was just going to take it for a crawl down the Kings Road, where most UK urban Ferraris spend their time! :P