The best times to prune your lavender are either after flowering in late summer, at least a month before the frosts, or late spring, after the frosts. I've tried both. I have a low lavender hedge several metres long and prune it with hedge trimmers. They flower better and sooner the next year with the summer pruning, but you have the advantage of the dried flowers through the winter with the spring pruning. Whichever you choose, it is a good idea to do it annually to stop them getting leggy.

The problem with pruning it just now is that if we have a frost soon it can get into the cut ends and kill back the shoot or sprig. So you might find your lavender suffers some dieback next spring, unless these warm nights continue for a few more weeks. The same issue with rosemary.

As others have said, if you cut back into the woody bits, there is a risk it won't reshoot, it seems to depend on the variety to some degree. But you've done it now, so keep your fingers crossed.