Manufacturers hp ratings can vary wildly as it depends what standard the manufacturer chooses to use. Some take the reading at the initial output at the flywheel, with and without things like the fan, alternator and other ancillaries. Obviously that gives the highest possible readout of their tractor power.
Others use different measurements with ancillaries engaged which gives a lower reading. It gets even more confusing when you start to include features like powerboost which many tractors now have in transport or pto mode.
When you get your tractor dyno tested they are testing the pto output so don't expect it to be the same as the manufacturers rated output, very few, if any, manufacturers use the pto output as their rated maximum power, it can be up to 15% less.
Another interesting site is the Nebraska test lab which was set up many years ago to test the sometimes extravagant claims of the manufacturers
http://tractortestlab.unl.edu/
You might be surprised at the results for your favourite tractor although most of the tractors tested are American and may not be available in the UK