Hello Eunice,
Sorry to hear your
Eucalyptus tree is a bit poorly. Eucalyptus need to be sited somewhere that is not subject to cold, drying winds. I know it is unlikely, in the current weather, but they must also be grown in compost that does not dry out, preferably soil based that will hold on to any moisture more efficiently than peat for example.
Is it possible that the tree has become pot bound? Gently pull it out of the pot and see how the root system looks.
Most Eucalyptus are not fully hardy to a proper U.K. winter, especially when grown in a pot; the roots unable to penetrate deeper, possibly frost-free soil.
If the uppermost foliage of a Eucalyptus appears sickly, you can prune Eucalyptus very hard to any healthy young shoots at the base. This should rejuvenate the tree, giving it renewed vigour, although this may be at the expense of the standard tree shape, resulting in a shrub rather than a tree.