Depends how much money it is and how important that money is to you.

If it hurts but you'd rather not have the time sink, drama, stress and effort of pursuing it, then drop it with lesson learned.

The second option, appoint a lawyer in England and if they say you have a case then let them pursue it for you.

I would warn you not to get too involved. Legal cases are extremely stressful and occupy a huge amount of head space. Try to just hand it to the lawyers and give minimal thought to it.

I've seen many people become totally obsessed with legal cases and who is right and who is wrong, and they can't think of anything else and alienate themselves from everyone around them because they can think of nothing except the legal fight. Don't do this.

Depending on who the company is, some people choose to make their quest to be paid public in an effort to shame the company into paying. It might work I suppose.

It's also worth just calling and calling and calling until you get to speak to the right people.

I've had clients who intended to pay but were under cashflow pressure and thus ignored me. My relentless calling twice a day eventually got them on the phone, at which point I asked to understand what was going on, and having secured a commitment to pay, I then relentlessly called daily from the day they said they would pay (they usually don't pay on that day). Start leaving detailed messages with every single employee of the company that you can get on the phone saying why you are calling. "Could you please get him to call me back ASAP regarding the unpaid bill of $X. I've left 30 messages and he has not called me back." I also explain to them that this is a small company and this money makes a real and tangible difference to my ability to feed myself and my family and could they please do their cashflow management with bigger companies who can absorb the pain.

And, if you want to play hard as a final resort, let them know you'll be very vocal with their client base and employees with personal phone calls until you get your money. Become such a thorn in their side that they pay to stop being annoyed by you.

Public posts on LinkedIn asking them to call you about the unpaid bill are also an extremely crass but possibly effective mechanism for getting them to sort it out.

Finally, don't be whiny or strident cause you'll sound like a crackpot. Always keep the story straight and bring it back to the certainty that you are owed the money. "We've got clear signed terms here, the work was done, the costs incurred, payment now 130 days past due, there's no reason this should not be paid". Never get angry when communicating with them about it because this will give an excuse to disengage and will lose you the high moral ground... keep monotonously consistently factual and emotionless with your message.

Another option you have is to sell the debt to an english debt collector. I know of one debt collector who employed a well-known former world champion boxer on salary and I have no doubt he made some personal debt collection visits - there's a fine line there but if these people have effectively stolen your money then why not spend 50% of it to make it their problem with an aggressive debt collector.

Is it a small company or large company? Maybe they have gone out of business? What do you think is the reason they have not paid?