It is a huge doubt I have, because I have seen on CAP 413, FL 200, transmitted as flight Level two hundred, and how should then transmit it on Europe (out of UK) and on The USA airspace?
Your confusion is understandable, since, back in 2016, a number of countries changed this phraseology as ICAO published the 7th edition of Annex 10. In the EU specifically, EU regulation 2016/1185 applies, which states that, generally speaking, each digit should be spoken separately, but with some exceptions:
Flight levels shall be transmitted by pronouncing each digit separately except for the case of flight levels in whole hundreds.
Example: Flight Level Two Hundred (instead of "Flight level two zero zero").
The altimeter setting shall be transmitted by pronouncing each digit separately except for the case of a setting of 1 000 hPa which shall be transmitted as “ONE THOUSAND”.
Example: QNH One Thousand (instead of "QNH one zero zero zero")
All numbers used in the transmission of transponder codes shall be transmitted by pronouncing each digit separately except that, when the transponder codes contain whole thousands only, the information shall be transmitted by pronouncing the digit in the number of thousands followed by the word “THOUSAND”
Example: Squawk seven thousand (instead of "Squawk seven zero zero zero")
The UK have obviously implemented this EU regulation, which is why you find the new phraseology in CAP 413.
“Flight level two zero zero”
would be standard pronunciation.
While everyone should use proper radio phraseology the system is not 100% perfect and you may hear other things on the actual radio.
4-2-9 Altitudes and Flight Levels
a) Up to but not including 18,000 feet MSL, state the separate digits of the thousands plus the hundreds if appropriate. EXAMPLE-
12,000. . . . . . . .one two thousand
12,500. . . . . . . .one two thousand five hundred
b) At and above 18,000 feet MSL (FL 180), state the words “flight level” followed by the separate digits of the flight level. EXAMPLE-
190. . . . . . . .Flight Level One Niner Zero
275. . . . . . . .Flight Level Two Seven Five