Lemon trees do lose leaves throughout the year but you should check the underside of the leaves just where they connect to the branch as you may have scale insect. They are small flat round shapes. If you have them you need to remove them, gently rubbing with your fingers should do it for a minor case. I'm not sure if there is a chemical solution available
Have you had a lot of rain or might you have watered the plant a bit too much? Citrus trees are best slightly under- rather than over-watered and if the roots get too wet they will start to drop leaves.
As MuddyFork says, the other possibility is scale insect. When my Kaffir lime was quite young we had a minor infestation, which we cured by dilligent application of a cotton bud soaked in meths to each scale. They are sometimes hard to see, so a magnifying glass helps. I also sprayed with soft soap diluted in water, in a hand sprayer. This old-fashioned soap product is available in virtually any supermarket in France (savon noir) but might be a little harder to get hold of in the UK. This seemed to help to get rid of any remaining adults before they went into the scale stage.
We do our very best to be as organic as possible, but there are agro-chemical treatments if you want to use them. (As Monty Don definitely wouldn't say! )