Hi Chirpy Pilot!
As you probably know all the airspace is regulamented (is that the word?) and the use of airspace obeys to certain rules, like odds or evens levels according to your heading, and there are some portions of airspace are for military only, others for oceanic entry, etc., so there are enumerous reasons for a flight level not being available.
Monty Nivo is right, but you can find that 300 foot (it`s 300, not 200) rule in the ICAO/JAR documents about RVSM. It is documented, and an undershoot or overshoot of your flight level by more than 300 feet it is mandatory to fill a report and send it to the appropriate authoritys, in this case, NATS, and the CAA
Cheers!!!!!!
