I think the Wool Board price by the sheet (the correct term for the humungous sacks), so take an average of what they find in each sheet.
Firstly, there is no tariff, you don't know in advance what you will get overall, and it varies year on year according to what they manage to sell the clip for.
You get paid an 'advance' on submitting your clip, which is the very least they expect to be able to get for it, and will get the 'balance' payment the following year once they've sold the clip and know what they can afford to pay out. Some years the balance is tiny, some years it's better.
The 'advance' payment tariff
is published, so you could always ask for sight of that before deciding what / how much of your clip to submit. But it's no indication of what the final payment would be, really.
If we send our whole clip, the last couple of years we've averaged about £3/sheep. Ours are commercial Texel type and Charollais type sheep, with about 10% being North of England Mules. Apart from the mules, our fleeces are outstanding for the type (though I do say so myself

- I feel entitled, having had so many handspinners now wax lyrical about our fleeces

)
When I take fleeces out of the clip for my spinning friends, BH charges me £5/fleece because of course I am taking the very very best of the best out, so may be impacting the average grading we get when the graders open the sheets.
You shouldn't send the discarded bits of the fleeces you've used yourself to the Wool Board - you certainly wouldn't get much if anything for skirtings. They want clean whole dry fleeces, properly wrapped, no daggings, no belly wool or second cuts. Yes they will pay more per fleece if they're properly skirted but we tend to just pull off actual dungy bits when we're wrapping for the Wool Board. The industrial processing can remove all the branding paints etc too, whereas for handspinning any painted wool will be discarded. (Raddle washes out, as does the spray can marker, but branding paint and the original type of rudd do not.)
The Wool Board do publish a chart showing the likely payment per kilo by breed, so you could ask them for that.
I have a spinning friend really wants to try a Suffolk fleece - if you get a nice one, please contact me

(Hoping we could arrange transport by TAS- and/or Ravel-courier!)