As with others in the comments, I am not a solicitor of any kind but this is an interesting question that I learnt about in a law class I had recently, where I asked this exact question.
It appears that this may be somewhat of a grey area in the Investigatory Powers Act 2016. This states, under section 3, that the "Offence of unlawful interception" shall be committed if a person "intentionally intercepts a communication in the course of its transmission".
It defines "Interception", in section 4, as "a relevant act in relation to the system" that makes any content available to a person who is not the sender or intended recipient.
Subsection 2(b) states that a "relevant act" means "monitoring transmissions made by means of the system". On the surface, this would seem to indicate that any monitoring is made criminal but in the relevant case law, "monitoring transmissions" seems to have been defined very narrowly as to only include proactive "acts" (I'm struggling to find the relevant cases on this).
If passive monitoring doesn't require any proactive "act" then I believe it can be argued that it does not constitute "interception". The only question remaining is if connecting to a wifi network is a proactive "act" or if it is passively listening to the network.