You can no longer join Essex library without living in the area.
They responded to my query about being able to join online but not borrow any books and said they've recently changed the criteria due to concerns from publishers. To be able to borrow books you have to convert your temporary online membership to full membership by visiting the library. From what they say, it looks like tighter controls might be put in place everywhere:
A recent change in arrangements concerning Overdrive e-books and e-audio services means that they can no longer be made available to people who have on-line temporary membership. This restriction has been put in place by our suppliers, in response to concerns expressed by publishers about libraries providing e-book material to users who do not live, work or study within their geographic area.