UK small claims courts.
https://www.gov.uk/make-court-claim-for-money
You can file your claim online if it's under GBP$100k (~USD$130k). There is a faster/simpler claims process if the amount is under GBP$10k (~USD$13k).
Usually this is enough, however if it gets to the point where they have a court hearing and your presence is required (unlikely), then give someone right of attorney to represent you (eg. friend, lawyer, etc.).