There's a reason the wife came down to say that the dh had told her that you had said yes - if the husband had come down, he would have had to have asked you if you would give him the wifi and you could have said no.
By sending her down, saying 'thank you for letting us use your wifi' they were sneakily manipulating you so that they had changed the default position from you not sharing and having to do something to actively share it, to the default position being that you are sharing and that you are having to say that you didn't agree to them using the wifi - which with typical British nervousness and reserve can be a difficult thing to do.
They know this - it's why they didn't come down and ask 'can we use your wifi' because that is the position that gives you the upper hand. They have changed it around to the position that they know gives them the upper hand.
Either put a note through their door or be ready to tell them that you can't share your wifi - you have looked at your wifi contract and you would invalidate it by sharing it with them, which you are not prepared to do. However, you are using Virgin and found them good/bad so do/don't recommend them and they took about 3 days/week/whatever it was to set you up and running after signing up with them. And that if they look on the comparison sites there are some very good deals at the moment in the run up to the end of January (no idea if there are or not, but there are always some deals around and lots of them seem to have a deadline of the end of January as the can use some spin of January Sale as their tag line to pull people in).
That way you're giving them the answer that you can't hare wifi, but rather than stopping there and giving them a chance to badger you, you're moving the conversation on to places that they can get wifi from, and talking about good deals available, so you're being helpful...
if they continue acting dreadfully, and if they continue to badger you, I'd talk to the landlords, just to say that your neighbours seem to have problems sorting out their own wifi and are trying to pressure you into giving them your code (it's not even sharing as I'm guessing that they're not offering to pay anything towards it, not that you'd want it as you don't want to share and indeed shouldn't be sharing, particularly with virtual strangers), but again it is another example of how they are being CF and manipulative - talking about sharing but expecting you to give the password out and pay nothing in return.
And just spotted that they are banging on your wall (started this reply earlier and got distracted when ds woke up) - definitely record it on your phone. Do you have a tv you could put on quietly for a few moments and show the volume on or something that you could do to show how noisy they are?
Started to make recordings and notes of what they are doing and when, starting with basic interaction over christmas, then talking to the dh at the supermarket, the dw when she came down to talk to you, the deliberate misrepresentation of the facts, the banging on the wall and so on - so that you can report them for the noise as as well as for harassment.