Nigel,
The latest amendment to ICAO Annex 6 (revision 12) contains a Standard for singles flying in IMC.
It is not as simple as retrofitting an autopilot, it has to meet compliance with Appendix B to Part 27 and the additional equipment Standard for flight in IMC.
As Hihover has indicated, it is not simple to replace VFR operations with IFR - it requires the appropriate certification, equipment, qualification, experience and infrastructure.
Whether this (economically) buys sufficient additional capability when operating to other than airfields is questionable. Even in the coming era of GNSS Point-in-Space procedures, the infrastructure requirement is way beyond that required for VFR flight.
That is why you will continue to see sophisticated SPIFR twins flying VFR (for HEMS, police etc.).
Jim