Above a certain weight limit (mass, for physicists) - between 7kg and 20kg models cannot be flown above 400 ft unless cleared by ATC (I think this only applies to controlled airspace - I doubt they expect you to provide your own ATC facility in the back of beyond), and you have to keep 150m from congested areas.
Above 20kg you need a CAA 'Permission to Test' exemption certificate, which lasts 12 months, before you can fly the critter. In addition, the model must be inspected during the build (there's a countrywide list of examiners apparently) to ensure it's constructed correctly, an examiner then supervises a test flight to check the 'pilot' is competent. The certificate okays named 'pilots' to fly the aircraft, all named pilots have to have flown a saitsfactory test flight with the examiner.
There's probably other bits and bobs involved - there's a Large scale aircraft club (LMA?) of some sort within the auspices of the national club (BMFA), and they all take the construction and proof of competent handling very seriously indeed.
Saw a Gloster Gladiator in an article, I think it was 1/3 scale, and looked quite capable of providing a 5 year old with that authentic 1930's 'wind in the hair, bugs in the teeth' experience.
Looking for a kinetic energy kill, an alternative is to buy one of the modern jet models instead - you can buy them ready made too, I've seen a few advertised...F15's and the like going for up to 15 grand a pop....I suspect one of them going full chat up an intake would do every bit as much damage as a 20 kilo DC-3....
Dave