Regardless of your own personal ability, THE LAW states that all new riders are restricted as described. The 'quite rightly so' is to prevent many new riders from thinking that just because they've ridden an RS125, they can go out and hoon around on an R1.
There are bikes out there that will do 0-60 in less than THREE SECONDS!!
You offer even the notion that a few goes on a mate's 125 means someone can handle that and kids will be blazing off and crashing out like there's no tomorrow. That would be a tragic waste of many beautiful bikes!
Credit where due, you have some experience, but not everyone who comes to the forum asking such questions will be like you. Most are NOT, in fact, so based on the initial information you gave, this is why people have been so.... harsh.
Either way, from 350cc to a 650 weighing 37 stone *is* quite a hefty jump and even after you get your full licence, it will a bit of getting used to.
Besides, UK roads are very different - In countries where people drive like loonies all the time you expect stupidly dangerous stuff to happen. Here, it will happen when you *least* expect it!!