If you fly IFR in CAS then you need a Mode S now and I don't think any further discussion of this is going to be useful.
Unless you want to limit yourself to the UK only and hope for some concessions on airspace access, but a UK-only IFR situation is a bit moot anyway since one can get around the UK in Class G, and nobody really cares whether you are VFR or IFR in Class G... - except ops at Class D airports.
VFR remains questionable; not (IMHO) because of the cost but because there are significant technical issues in installation in many types, so some exemptions will need to be worked out.