I would imagine at this time, it is either flowering or about to flower soon.
Personally, I'd leave it until next spring until you prune, for best results. All the Best T bird+
Cut it back well after flowering round about late September. This will improve the density and make it more bushy instead of leggy...I just go over mine with the shears so it's all level.. then I just give them another light prune in the spring. I keep the lavender stems though and hang them up to dry and then use it in lavender bags or sprinkle it in the linen cupboard .
Best hard pruned after flowering to keep it bushy, you just need to leave some green shoots, don't cut right back to the brown old wood (about an inch of green will do). It will keep it bushy and you shouldn't really need to prune again in spring. If you miss pruning it after flowering you can prune in spring instead, but don't cut it back too hard. good luck with it . H.