In the UK, top price broadband maxes out at about £50 per month

And, if the organisation was of a sufficient size to get a volume discount, they might be able to bring the cost down even more. But that would entail the employee accepting the employer's choice of supplier.

using someone else’s unsecured WiFi. Is that theft?

No. To be theft, one has to dishonestly appropriate someone else's property (with the intent to deprive them of it permanently).