How secure is it to receive a verbal offer for an academic position in UK?
An email offering you the job is not a 'verbal' offer, and although it's not as binding as a contract, I would certainly consider it secure, particularly from the head of school.

Note that the contract may come via snail mail and if you're on a different continent that could take a while. Perhaps you could try to get a confirmation from HR that the contract has been sent.