The reason I’m hesitant about positively IDing this one is because there are two species of micro moth whose larvae produce very similar mines on bramble (Rubus fruticosus agg.) leaves. One is the Glossy bramble pigmy moth (Stigmella splendidissimella) but the more likely in this case is the Golden pigmy moth (Stigmella aurella).
Though it’s very common and widespread in Britain, I’ve never seen this tiny moth (images on the UK moths website here) but the mines its larvae create on bramble leaves are everywhere I look. As you can see, the mine starts out small but, as the larva within chews and chews, so it widens its mine to accommodate its expanding girth.
You can see if the inhabitant is still at home by holding the leaf up to the light. Most of these seemed to be empty, as you can see from the images below (the central black lines are the frass). I’ve never actually opened a mine to have a look at the larva when it has been inside but, if I did, I would see an orangey-yellow grub with a brownish tinge to its head (there are images on the Bladmineerders website here).