Fraud by false representation, contrary to sections 1 and 2 of the Fraud Act 2006:

2. Fraud by false representation

(1) A person is in breach of this section if he—
   (a) dishonestly makes a false representation, and
   (b) intends, by making the representation—
      (i) to make a gain for himself or another, or
      (ii) to cause loss to another or to expose another to a risk of loss.

(5) For the purposes of this section a representation may be regarded as made if it (or anything implying it) is submitted in any form to any system or device designed to receive, convey or respond to communications (with or without human intervention).

The (mis)representation is that an HTTP request (say) is made by an authorised user of the WiFi network in question, and the (mis)representation is made when the request reaches the ISP's server.