So I was able to get a pack of 2 heatbreaks for £4.99 on Amazon and asked a university technician if it was possible to reduce the M7 thread to M6 and the length above the hex to 6mm. We were worried if the material was going to be too thin, but there was more than expected as the steel tube is a lot shorter (and a bit smaller) than expected. The 4mm inner bore goes much further into the heatbreak than I assumed:

I applied the changes to the heatbreak CAD model and then digitally assembled the legacy 4-in-1-out Machined Coaxial Hotend and thermally simulated it. 

I'm going to use the CHC Pro (possibly insulated with ceramic wool) I still have from #SecSavr Sublime [gd0036].

I also got some PTFE thread seal tape, which I believe is the cheapest single thing you can buy on Amazon Prime: