I live in the UK and I have a very large maple tree in my garden and just noticed it has white spots on the trunk and some branches.
Any idea what it could be and what I can use to cure it?
Most likely horse chestnut scale, common on large and small Acer varieties in the UK, as well as lime and horse chestnut trees. Although it's unsightly, on large trees it's impractical to treat, and won't kill the tree anyway. On smaller trees, you can use disposable cloths moistened with a little methylated spirits to rub over the affected woody parts, rubbing off the scale, then using a cloth moistened in ordinary water afterwards, but this is difficult to do on ridged bark, though works well on smaller Japanese maples. More info below, with image so you can confirm (or not) that this diagnosis is correct
Now you've posted a photo, that confirms horse chestnut scale.