I’ve no experience with Gritstones but I can share my experience with 400+ Swaledales running on very very poor Northumberland moorland.
We put the first timers and some older ewes to a Swale tup for replacements and the rest to BFLs. We treated the girls the same whether they went to the larger tup or were bred pure.
Everyone ran together on the moorland until scanning, which was about 8 weeks before lambing. No hard feed for anyone until then. Everyone got ad lib hay and or silage from about Christmas onwards. We gave them minerals, either drenched them with a top quality chelated drench such as Ovithrive or had the Crystalyx red buckets out throughout, or one year we used a powdered mineral mix from PK Nutrition. Unless they got a drench containing chelated copper, they’d get copper needles mid-pregnancy.
At scanning they were split - singles and fat twins in one group, would get no hard feed unless they seemed to be losing condition. Twins the biggest group would get a pound a head a day (of an 18% ewe roll) from scanning for four weeks, then a pound and a half a day. Very thin ewes and any trips that weren’t rolling fat got better ground and a little more feed than the twins.
We had learned the hard way to not feed singles unless they were practically emaciated.
I don’t know what type of ground you have, and the Gritstone is a chunkier animal than the Swale. But my instinct would be to make sure they have minerals your ground lacks, and to hold off hard feed until 8 weeks before, then feed to condition. Split them into ‘thin’, ‘fit’ and ‘fat’, and manage accordingly. I think I might use less of an 18% feed rather than more of a 16%, especially in the last four to six weeks. You’re wanting milk and milk needs protein. Fatties onto poor ground, and the least nutritious hay, skinnies on the best ground with ad lib top hay. But I’d ask other Gritstone breeders in your area, they know the breed and the local consitions.
One of the reasons people feed hard food in late pregnancy is that the lambs take up so much space the ewe simply cannot eat enough forage for her nutritional needs. But that’s more an issue when using a meaty tup, especially of a larger breed, or where both ewe and tup are meaty, and shouldn’t be such of an issue when breeding a hill type pure.