Foam is the obvious even the FR type, or possibly glassfibre rope, or fire cement, or make up a lightweight mortar with vermiculite, point up to finish, and possibly use a stainless type collar to cover the fill. My experience of the UK building supply industry is that generally they are disinterested in such finishing things especially if aesthetics too are involved. Have you tried the flue manufacturers for collars? Do soil vent pipes have to penetrate the outer wall of the building?

Lots of stuff available an02ew. Rigid fireproof boards, refactory linings, vermiculite sheet. All intended for high temp applications, furnaces etc. Try www.kitsonsinsulation.co.uk they have UK wide distribution depots. There are others, some even have basic cutting facilities. High grade stuff like this isn't cheap though and you may have to buy a full 8' x 4' sheet.