You should pay no more than an extra fiver to have your skin back.....in fact many abattoirs do not charge extra!
You need to collect it as soon as possible and no longer than 3 hours after slaughter. Get skins home and place wool side down on a pallet under cover eg in a garage and then its 1kg salt per skin.........and thats just a start! Google Fenland sheepskins, ring them and ask for June Tinnion (well in advance of taking your sheep to the abattoir!) She will tell you what to do! Then post them to June and they will make the most glorious sheepskin rugs you have ever seen that you can sell for in excess of £50
IF you have Jacob sheep, coloured sheep of any other description OR wiggly wool sheep like wensleydales, about £80 is easily obtained!
I sell all my skins and those of other smallholders.....they are very valuable so do not let them go!!!
Please get in contact with me if you need more help!