If Celtic Musif was a limited company the Company House website www.companieshouse.gov.uk will tell you if it has been struck off or liquidated. Other records there may show you whether there was a liquidator appointed or even perhaps if the rights were assigned out in the course of liquidation. If not they may be bona vacantia and an approach to the Official Receiver (or is it the Treasury Solicitor) department of bona vacantia might enable the rights to be bought.

As far as I know the issue of whether non-UK copyrights owned by a UK company devolve according to UK law or the laws of the relevant foreign places, when the companies cease to exist has not been thoroughly litigated although there was a case a few years ago the name of which eludes me right now which might indicate that the laws of the foreign places applied.

US law (US corporations) I'm pretty sure in most states the rights automatically go to the shareholders.

On the UK front it might be worthwhile re-reading the Elton John and George Michael cases.