[contingencies]

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.).

[CyberFonic]

This is probably the simplest approach. Of course it would be considered fiscally high risk to let any client rack up more than GBP$10k in outstanding payments.

[rahimnathwani]

This might be hard if OP is outside the UK. If the client responds to the claim disagreeing (i.e. saying they don't owe money), then the next step will be a court hearing. The amount in questions is likely less than the time+money it would cost OP to go to the UK to attend such a hearing.

"You might have to go to a court hearing if the other person or business (the ‘defendant’) denies owing the money and you disagree with their response."

[az123zaz]

I ended up filing with a debt collection agency in the UK. As an international client I lose 35% of whatever they collect but it still is worth it to me. The nice thing is that there are no other fees.

Funnily enough the client became very responsive after they found out we had legal representation in the UK. Still have not seen any money in the door yet.