How secure is it to receive a verbal offer for an academic position in UK? I recently accepted academic offer from a UK university. I comminicated wtih the head of school via phone and email, and I have not yet received the official offer letter. I am wondering how secure this situation is? In particular if this is a good time to decline other offers (I feel bad to let them wait too long and decline their offer eventually)? Or should I wait until receiving the official offer? Thank you for your input in advance.   

I saw a similar question, but that is for postdoc positions and I suppose it is not exactly the same.
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.