I think the JAR/UK CAA rules are slightly more complex. The same basic rule applies that for aircraft with more than 19 seats, 1 cabin crew member is required for every 50 passenger seats, but there are type-specific rules over and above that, generally for larger aircraft.
For example, the British Airways B767-300ER SEP manual states;
The ANO requires 1 cabin crew member per 50 passenger seats, but in the case of the 767-300 a special requirement has been imposed which specifies a minimum of 8 cabin crew regardless of the number of seats or passengers carried.
So while a US carrier may be able to operate a B767-300 with 5 cabin crew, the legal minimum for a UK-registered B767-300 is 8 cabin crew. UK airlines normal crew complemets are as follows;
Britannia - 8
MyTravel - 8
Air 2000 - 9
British Airways - 9-11