You get a tiny advance once they have the fleeces, and then the rest once all the wool has been auctioned through the year, tho they do give you an estimate.
I think from memory that I might be looking at about £40 for the 20 from last year NB with Shetlands, moorit brown ones pay twice as much as grey or mixed colours, and white ones are in between the two. You get a deduction for badly marked or dirty and daggy fleeces so there are separate rates for that and you get a grading sheet back - you just have to fold it up and sew it into the plastic wool sack they provide.
Less desirable wool from some other breeds can be half or less of the Shetland rate. generally wiry fleeced breeds don't get paid very much at all as its only useful for carpets and that's a small market these days and mostly covered by imports.