Depends on whether you mean legally or morally. I don't think that morally anyone would have a problem with you keeping it, but at what point does it become morally unacceptable, £1*, £5, £20, £50, plain brown bag full of notes?

Unfortunately laws have to be written, and they either get written with woolly language like 'acceptable' and 'reasonable', or they get an absolute limit, and in the world of IT, any absolute figure is going to look silly in a few years.
Or you just write 'you can't do this', and although most people will get away with it, if to some policeman that doesn't like the look of you, or being in the wrong place at the wrong time for you to get done.

It is really hard to write a law saying 'you can't intrude into someones network, unless you want to just check your email, and it is only briefly or in passing, and only if they have it open'



* Obviously finding a $1 where I am would be unlikely and also completely unhelpful! ** Luckily with the exchange rate now, just multiply by 2.
** actually thinking about it, not so useless, I am in the states for a 2 week holiday in september!