I expect AWS and possibly Track Circuit Block would be required for higher line speeds and I doubt many heritage lines want that.
You've pretty much got it, but I do wonder just how many times this question will get asked on forums!!
Preserved lines are 25mph maximum, for all train movements. Freights, LE, run pasts, enthusiast events, everything. The only exceptions are a couple of designated test tracks, when being used for that purpose only, and I believe a derogation for demonstration TPO trains at a couple of locations.
If you want to have a higher speed, then you need TPWS, AWS, maybe OTMR, (depending on the speed envisaged), centralised door locking, or to negotiate an alternative system, as with the Mk 1's used to Whitby. The infrastructure would need looking at too. Maintenance regimes and documentation, signal sighting & spacing, etc. TCB is the one thing that may not be required! (There's plenty of more primative signaling still extant on the network). Then crew competence would get put under the microscope. Either psychometric testing, or a documented experience based system, as has been used for Whitby running. That's the example of what's required, and it has involved a lot of time and money.