The Abattoirs, will probably add a extra charge to your slaughtering costs (for you taking the skin back), as they will have aloud for the skin cost to be taken into account when calculating the price for your slaughtering.
Last time I looked into it the abattoir's get between £2 and £4 per skin from the tannery, this depends on market forces , time of the year, quality of the skins and also the number they have for the tannery.
so in theory they will charge you the £2 or £4 extra on the bill.
If they don't charge any extra - well you are doing well