Thanks to Sue, one of my blog readers, for asking and making me realise that not everyone that reads my blog will have all the tech. knowledge.
There are a few significant reasons why it is desirable to stay under 3500kg.
Firstly there is liecencing considerations, If you passed yout driving test pre '97 license you will have C1 category to legally drive it. After that you need to take another test to drive over 3500kg
Then there are downsides that limit speeds on dual carriage ways and single lane roads, and if you go to into Europe you will be barred from quite a lot of towns and villages who operate a strict max. 3500 Kg access limit.
Also when you reach the age of 70 you will need a medical by your doctor which will allow you to continue driving anything over 3500kg.
Another consideration is specialist recovery also comes into play, RAC & AA etc. won't be able to recover anything over 3500kg with their standard vehicles. This requires specialist (expensive) recovery should you breakdown.