What is damaging my lettuce leaves?

That's a really interesting one, I've not had this on lettuces.

You might have two pests at work here, one making the trails and one making the dots.

My first thought on the trails was a leaf miner, but
- there aren't many leaf miners in the UK that attack lettuces (although see Lactuca [Asteraceae] in Leaf and stem mines of British flies and other insects)
- the trails seem to be ignoring the leaf veins so they are probably on the surface of the leaf rather than inside.
So I suspect the trail is a baby snail. If you sneak up on the plant at night with a torch you might see it in action.

Now the dots. Your picture isn't detailed enough to show: are they like blisters or are they little holes? Whatever, you need to find the culprits who I suspect are working at night.

As for cures, I would concentrate on hygiene. Clear away those dead leaves on the compost and try and keep the top few millimetres of the compost as dry as possible. Water from underneath if you can to facilitate this. Check all round and underneath the containers to see if there are any baby snails hiding.

Good luck!

They're leaf miner trails,the larvae tunnels inside the leaf,at the end of the trail you can usually squash the tunnelling larvae,if you leave them they can pupate in the soil. The mines reduce the plants ability to photosynthesis which might be the cause of the pale spots? Yellow sticky traps can help catch the flies,but keep a lookout for the trails & cut those sections off the plant,rather than losing the whole leaf. I've had them on a few different plants this year,at the moment they're on my rocket leaves.
http://fera.co.uk/plantClinic/docume.../liriomyza.pdf