Here's one BBC article that talks about it: http://news.bbc.co.uk/1/hi/magazine/6960304.stm
> The Communications Act 2003 says a "person who (a) dishonestly obtains an electronic communications service, and (b) does so with intent to avoid payment of a charge applicable to the provision of that service, is guilty of an offence".
Seems pretty clear. A person is paying for hotel access; notices their Apple-device using colleague is not paying; spoofs the user agent; - these seem to cover all the points of dishonesty with the intent to avoid paying for a service.
Even if the hotel is in a country that doesn't have that particular law there is probably some fraud law that covers the actions. (I realise now that I should have mentioned the fraud because that's the more serious offence and it seems some people missed my point).
To get charged under this law, you would have to be accused of the unauthorized access of something on the router itself, since that is the only relevant computer you aren't authorized to use. The charges against you would have to be based on the theory that sending packets with a spoofed MAC address or user agent is accessing the routing program and tables on the router in an unauthorized manner. That argument delves much deeper into the law than just the first clause.