Not sure how you got a RAS since Southampton is Class D and you cannot get an RAS in CAS.
If you were going to land there you would get radar vectors to the ILS, so you fly as directed until localiser established and then track the GS.
Or, if you want practice, you fly the procedural ILS or whatever, and it is 100% legal to use a GPS (in OBS mode, etc) to supplement the VOR information. In the private flight context, the ANO specifies equipment to be carried. No law states what equipment should be used.
Hi,
I had obtained prior permission to fly to S'ton. Shortly after Compton, in IMC, I made contact, requested and received a Radar Advisory Service - which became Radar Control only on entry into the zone. As always I was on my own navigation prior to zone entry. Since I was already more-or-less on the runway-in-use centre-line I elected to do a VOR/DME approach, permission for which was granted so vectors were not necessary.
I believe that by law one is only allowed to fly a 'published' instrument approach. The legality of using a GPS to fly a VOR/DME or ILS approach I leave to others who know the law better to comment on. I guess as long as the VOR is working & correctly set for the approach one could use the GPS as long as the VOR is there as backup if needed.
Anyway, I think we have deviated from the original thread so I shall let it rest.