It has never been compulsory to vaccinate against BT in England, and was compulsory in Scotland only in the first year. We vaccinated last year as well, as cattle breeders were importing to our part of Scotland from infected areas of the Continent.
I believe there is to be a review of the situation this Autumn, so wait for advice.
The usual time to vaccinate is in the midgie-free period ie the winter. This means for many people that their ewes are in lamb when they have to be vaccinated. Because it was compulsory in the first year here, we had to vaccinate in-lamb ewes, so we did it right in the middle of the 5 months gestation, when the pregnancy is most secure ie mid-Jan. We did the same thing last winter, with no ill-effects. Lambs will have some resistance passed on by their mothers for the first few weeks of life and can then be vaccinated from 1 month onwards.
Blue Tongue is such a horrible disease, causing great distress and frequently death to the animals and often leaving them infertile, with no compensation from the government, that for the sake of a few pounds it is worth vaccinating if there is any threat at all of BT reappearing in Britain.