clutches of the Catholic Church
They wouldn't be involved; they don't operate the undertakers or the mortuaries..
what the processes are etc.
Your family in the UK would need to contact a local undertaker in their town who would bring your body over (they are used to doing this). It is expensive, however it's possible to be cremated (without ceremony) in Poland and have your ashes sent over instead.
Any undertaker (nowadays the UK ones mostly belong to large chains) is used to managing this. For an insurance policy, any standard UK one would do. Specific ones for funerals don't cover international issues, however a normal one is fine providing it pays the money into your estate; the cost of the funeral is then taken from the estate during probate.