What should I do to care for an olive tree when it gets cold (here in the UK)? The tree is about 3 feet tall and I have access to a greenhouse (unheated).
You should transfer your tree into your greenhouse, but try to select a shady but not dark place.
Irrigate the tree only a little bit, the substrate in the pot should not dry out. Check the temperature in your greenhouse regulary, it should not go below 10°C the whole day for a long period (f.e. in december/january/february), and you should ventilate your glasshouse once a week. If the temperature goes down too much, you can use jute-textiles or similar to wrap the tree.