It's a bit complicated. The organiser will allocate qualifying certificates based on the number of competitors in the class. Think it's something like one place for every 5 competitors or something. Then the highest placed competitors who have not already qualified will get the certificate.
Each centre then runs a semi final (round about June time) at which there will be one place in the final for every I think it's 15 competitors. You can go to as many semi finals as you want and UKriders also give out wild cards to the final.
Did you compete at Croft yesterday Hayley? I know you posted a while ago saying you were thinking about it. Their January dressage is a UK riders qualifier