Hi Smee
We are finding it quite a bit harder to get our slaughter lambs booked in at Griffiths this year, was much easier when Daysdrove in Bishops Castle was operating too. We had to arrange for another butcher to take the carcases to butcher, Griffiths deliver to quite a few butchers in the area, and then Griffiths were able to just do the slaughter, otherwise we would have been waiting until after Christmas too. Unfortunately now it is getting closer to Christmas you may find it harder to get them booked with a butcher for slaughter, it may be worth considering just sending 2 off initially if they could be taken, and leaving the other 2 until after Christmas.
At least with Zwartbles you don't have to worry about them going over fat, you can take them up to a much higher weight then many breeds and still have excellent meat. We just collect our meat yesterday, unbelievable we had a couple of carcases over 38kg, they are usually closer to 23kg or so, the butchers said it must be the breed as they still had excellent meat and just the right amount of fat! We had a lot of happy customers yesterday with some bonus Kgs in their half lamb bags!
We have also had Eurolamb in Craven Arms take some lambs for slaughter for us, unfortunately they won't butcher or deliver and we had to take the carcases to the butcher next door for hanging and then butchery. It would be worth trying Welshpool if you don't feel it is too far for you to travel, at £15 per lamb they are amazingly good value. We use a couple of old trailers to provide portable sheltered lamb creep feeders/shelter when the lambs are still on their dams so most of our sheep are quite happy to be in a trailer, so they should be fine travelling!