Barry, it's not that simple. When sending anything through the post you have to anticipate it will get thrown about a bit. When handling 80M items a day, this can't be helped. Inevitably, some things will break or leak. When you are building a model aircraft (to an excellent standard, I might add), you know what you are handling. A postman will have no idea, and if you think about the different things that get sent through the post, I think most of us would have some sympathy with them. Add to this the damage that paint, for example could do to other people's mail, or the multi-million pound sorting machines, and these restrictions seem reasonable.
Here is a list of items prohibited by Royal Mail. Are there any more that represent "the curse of 'elf and safety"?
http://www.royalmail.com/personal/help-and-support/Tell-me-about-Prohibited-Goods