If you are are on instruments, using dual VOR with DME to navigate and receiving a Radar Advisory Service why do you imply that including a GPS in your scan as well is a good way to be spending your time? It it all starts to go pear-shaped or something packs up it will still be there to fall back on. I was responding to the suggestion in a previous post that ADF and VOR are not the instruments to be used for navigation in this situation. I don't agree with the earlier post.
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.
If you were merely tracking via SAM then you would get RCS once inside their CAS, and perhaps a RIS outside, although one could go for an RAS too (do you pilot one of the cheap airlines which flies in Class G? - they often ask for a RAS as per their OPS manual). And you can fly via SAM using the VOR info, the GPS (with SAM as a waypoint), or both, as you wish, all 100% legit.
Sensibly, SAM would be tuned in anyway.