In the UK, they probably mean small claims court, google it. There is a process online for that they may try, but I suspect they may be bluffing to try and get money from you. Realistically though, they are the ones that broke the (presumably) verbal contract by ending the arrangement early. You could try Citizens advice for more info on your rights and about mediation.
Could you just offer to pay for the 2 weeks at the rate initially agreed, minus what they owe you? Though sometimes just paying the money and blocking them may be the least stressful thing to do, even if it doesn’t seem right.

Was there a written agreement? Either way, I’d wager that they will not actually move forward with suing you (though unfamiliar with the UK’s legal system, I imagine there would be fees involved to do so making going through with it not really worth it for that amount). In your situation I’d probably do what @Tui suggested — give them some money for the time you did spend there minus what is owed to you. Then deleted/block and move on because yikes! These are not nice people.