Originally Posted by Mehitabel
and i think the romsey one is the 'dorset county show' for example.
It could be, if it wasn't in Hampshire
Dorset county show is Dorchester (I think it used to be called Dorchester show)
Romsey is an Agricultural show, which are generally a slightly lower level than County shows.
As sais above, county shows have lots of the big qualifiers, they also have lots of other competitions (for other animals, tractors, produce and sometimes things like shoeing and woodchopping), stalls and attractions. Agricultural shows are like county shows, but on a smaller scale, and often have some unaffiliated showing classes alongside affiliated ones.
There are some "horse only" shows which hold qualifiers too. These aren't county shows (because they don't have the other attractions) and are usually held by the big showing societies, like the British Show Horse Association has several large annual shows in different regions of the country, and the British Show Pony Society is divided into areas, each one putting on their own shows.
There are smaller shows which hold more minor qualifiers (for society championships, rather than for the horse of the year show or Royal International), and some which hold qualifiers for regional societies, like the south west ponies association etc. Then there are "local" shows, which vary from big to very low key.
Most people start off at local level, and if they do well, progress to doing some agricultural shows and/or affiliated classes (with qualifiers for society championships). If they hold their own, they'll try some Royal International or Horse of the Year Show qualifiers, which are usually held at county or larger agricultural or society shows.
I hope that makes a tiny bit of sense!