|> Light frosts are no problem. I grew up next to an olive orchard in
|> Corning, California, a town that billed itself as Olive City and has an
|> olive painted on it's water tower. Winter temperatures down to the low
|> 20's Fahrenheit were not unusual. Olives can take temps down to around
|> 15F or -10C. You do realize that your olive plants are going to be
|> fairly big trees in a few years?
Not in combination with wet and/or cold days, they can't! He should
be OK, but -5 Celcius is as low as you can risk in combination with
wet or if the days following frosts don't warm up much.
There are lots of plants hardy to -15 Celcius in the Mediterranean
or USA that are hardy to only -5 in the UK, and many parts of New
Zealand are more similar to here than there.