This is not due to cold damage on the flight. I don't know if your figure of 7C in a plane hold is always true, but it certainly isn't allowed to freeze. There is never a problem on a short flight from Italy to UK for any citrus - I've often imported them this way.
It has been far too hot in a greenhouse in the UK for the past week, unless it is fully shaded. Although pummelos like tropical conditions, temperatures over 45C will be too high unless you continuously damp down the greenhouse floor. I remove all my citrus trees from the greenhouse from mid-May to mid September and they do far better outside.
I suspect your leaf problem is due to excess sun and heat. Tender new growth and recently fertilised nursery plants are likely to be worst effected. Those leaves won't recover, but the plant should be OK if you keep it outside and shaded for a few days.
Citrange/Mike - Buckinghamshire, England
