From what i've read about farming in Ireland, the amount of lamb exported into the Uk has halved.Brexit has had nothing directly to do with the sheep trade.
Firstly for trade apart from increased paper work it has never happened. We continue to comply with all of the EU regulations for sheep meat so we effectively have a free market.
As has been pointed out the weakness of sterling has been a help in increased prices, but is a double edged sword as inputs are much more expensive.
Sheep numbers have declined here and in other parts of the world for different reasons. Drought, ageing farmers, poor profitability, low wool prices, and for a while Swine Flu in China etc. So causing a supply and demand imbalance.
Sheep numbers take time to increase, unlike pigs and it will be 4 or 5 years before we start to see over production that will reduce prices significantly. So if these breeding sheep this Autumn are over £200 they may be £50 culls in 6 years time, so once again we will have depreciation.
The single greatest concern at present are the shenanigans over the Irish trade deal, if the EU get really fed up with our politicians reneging on an international deal then overnight we could see the threatened tariffs being put in place. Then watch what happens to the lamb price!!
We also don't yet have any BCP's (Border Control Posts) in France at our closest points of entry so we are unable to export any stock alive. Ironically we have none on this side of the Channel either so there are no checks on imports!!