In domestic European airspace there is generally no different separation standard between aircraft based on their navigation capabilities. This is because they will either be separated or monitored by using radar (which is not dependent on aircraft fit), or by route structure (which is dependent upon the RNP value designated for the route).
Radar separation varies from 2.5NM on final approach to 10NM en route, or sometimes even greater.
Route separation is mostly based on BRNAV at the moment, which is RNP-5.
For radar monitored parallel RNP-5 routes, ICAO suggest a minimum distance between the route centre lines of 10NM-15NM. In practice, I think our UK designers go for 12NM. If it is that spacing or more, then monitoring is all that is required. If it is less, then the routes are not deemed separated and our rules usually say that appropriate radar or vertical separation must be provided.
For RNP-5 routes which are separated procedurally, and the only 'surveillance' is from pilot position reports, the ICAO recommendation is 16.5NM between the centre lines for uni-directional routes and 18NM for bi-directional routes.
PRNAV equates to RNP-1. I have not seen any formal confirmation yet, but previous concept documents I have seen were suggesting that for radar monitored RNP-1 routes, the spacing would be 7NM and for procedural routes would be 10NM.
In TMA airspace, even lower RNP values might be achievable, but by interpolation of guidance material, would still only offer a slightly decreased route separation value, which in any case could never come below the separation standard value of the surveillance equipment being used. If you went for RNP-0.3, the minimum route separation for routes which are monitored by surveillance would be in the order of 3.6NM for uni-directional routes. You would then of course have to assess the risk of being able to pass a corrective action in the event of a deviation between aircraft only 3.6NM apart in enough time to prevent a collision. Fortunately that kind of decision is way above my pay grade
