it's not theft; that would require dishonest appropriation of property belonging to another with the intention permamently to deprive that other of posession. It is unlikely that in these circumstances any of the elements of appropriation, property and intention permamently to deprive are present. (For dishonesty, see below.)

(An exception might be if the unauthorized user knows that internet access via the router in question is paid for on the basis of up to X MB of content downloadable per month; in those circumstances, the unauthorized user would be appropriating part of a chose in action, namely the contractual right to download X MB in a given month, with the necessary intent.)

However, it may constitute the offence contrary to section 1 of the Computer Misuse Act 1990.

Section 35 of the Police and Justice Act 2006, which takes effect from a day yet to be appointed, increases the maximum penalty following summary conviction to 12 months imprisonment and/or a £5,000 fine, and provides for a maximum penalty following conviction on indictment of two years imprisonment and/or an unlimited fine.

Another possibility is obtaining services dishonestly contrary to section 11 of the Fraud Act 2006.

So far as dishonesty is concerned, the leading authority is R v Ghosh [1982] QB 1053 (CA), which laid down the following test:

In determining whether the prosecution has proved that the defendant was acting dishonestly, a jury must first of all decide whether according to the ordinary standards of reasonable and honest people what was done was dishonest. If it was not dishonest by those standards, that is the end of the matter and the prosecution fails.

If it was dishonest by those standards, then the jury must consider whether the defendant himself must have realised that what he was doing was by those standards dishonest. In most cases, where the actions are obviously dishonest by ordinary standards, there will be no doubt about it. It will be obvious that the defendant himself knew that he was acting dishonestly. It is dishonest for a defendant to act in a way which he knows ordinary people consider to be dishonest, even if he asserts or genuinely believes that he is morally justified in acting as he did.