They have an aircraft which they own, and which I am licenced and insured to fly. What (if any) action could I legally take with regards to impounding/taking possession of this aircraft?
You may be insure to fly the a/c but if you take it without their permission it's no different from taking a car that you are insured to drive but don't have the owners permission to do so on a particular day - you're stealing it. You'd be breaking the law, the insurance wouldn't be valid and Mr Plod would be whistling in your ear fairly loudly.
Getting a lien on the aircraft would be difficult and expensive & might not achieve a great deal.
As far as I can see, you have two solutions:
If the debt is less than £5000, you have the option of taking it through the small claims court, but my favoured one (even if the debt is less than £5k), would be to engage a good solicitor and get them to serve a winding up order on the company.
That way you'll either get your cash or have the satisfaction of sinking the bastards.
Either way, pursue them and don't let go ntil you've got what you're owed.