andrewc - first the VFR-only use of GPS at, say, PPL level. In my opinion you should draw your route on the map and calculate track and groundspeed in the conventional manner. In the ac, enter the GPS route with turning points. Select the data fields to give DTK, GS and ETA plus have the mini-CDI bar displayed. When you are airborne, start your navigation and use the GPS to cross-check your pre-flight work. You can keep an eye on the mini-CDI from time to time, but your primary navigation is by map and time, backed up by GPS.
However, if you have an IFR-approved GPS with all the relevant requirments, things will be different. As far as I'm aware, the UK hasn't published any GPS non-precision approach procedures yet though.
The 'day job' ac uses basically twin INS, one of which is GPS blended. We do not use GPS-only for LNAV, we use the best FMS-derived steering solution to navigate between database or manual waypoints. That's because the GPS element does not have RAIM (astonishingly), nor does it have adequate stability to be coupled directly to the autopilot.
Other ac with infinitely more modern GPS/FMS would not be so hampered, but my point was that not every GPS is suitable for IFR navigation and approach - although it might seem to be very accurate, there's still a chance of it going out to lunch at an embarrassing moment unless it has the appropriate mandatory certification.