Not sure that drought's the explanation, particularly if your tree has been in the ground for more than 3 years. the new leaves are prone to frost damage, but even that wouldn't account for this because the top leaves are obviously unaffected. As you live in Kent, where the soil is inclined to be chalky in parts, it's possible it doesn't like its situation - they prefer acid soil. Other causes of die back could be bark damage, particularly from strimming or mowing - otherwise, this tree is particularly susceptible to honey fungus, but I can't think of anything else which might be causing it. Certainly, if you are still in a drought situation, it would be worth giving it a good soak.