It's very expensive to hire a lawyer for this, and it's not an easy task either for the company to prove what they claim you did. It's also one thing to sue someone and win a case, and totally another to get paid. All this costs a lot of time and energy, and I doubt you did (if you did) enough damage to be worth all these procedures. As mentioned in the other answers, it's probably bluff.
Maybe this contains enough information to reassure you, I doubt they will really sue you: http://findlaw.co.uk/law/dispute_resolution/litigation/basics/500437.html
One thing about the fact they accuse you of accessing their server: YOU can turn these accusations against them and should in fact mention how you find it terribly hurtful to your reputation to be accused of what are serious criminal offenses. Technically, you were given access to these. No one can call someone else a liar or a thief without strong evidences because this can easily be turned against the accuser. So instead of being scared, you should in fact have the mindset of anyone respectable who is accused wrongly. You could remind them that these are serious accusations and from what I understand, that person is also sharing this with other people. If you were living in the same country as this person, it would be very easy for you to start some legal procedures.
As for the money they still owe you, you could simply keep asking for what is due and you can always send an official letter. Don't be rude with the company, keep a door opened, they might actually beg you in a few weeks or month for your help and that's when you'll have the opportunity to get back what they owe you. You don't seem to be too worried about this $100 though.
But as for the company in UK, you also have little hopes that the law will help you for such a small amount. If you can't get paid, you should probably consider moving on and keep your energy to find new projects instead. For your next projects, do not deliver any final files before getting the final payment and never work for weeks or months on projects without being paid at least every 1-2 weeks. Some regular clients can be trusted, and you can always offer rebates on your project by offering them to pre-pay the full amount for a project. Even if you have a contract, it's not a guarantee you'll get paid; the real guarantee is actually receiving the money as you ask for it :) This way, if the company resists in paying you, you can always simply stop working on their project and invest your time in a project worth your skills! Usually when you put a project on stand-by because you are waiting for a payment, and mention it, the payment comes in verrry quickly! Don't be afraid to pause some projects until you get paid.