Since I provoked a number of comments, probably worth qualifying some of what I said
With the 20s, 37s and 86s - they're over 40 years old now. Spares will become more and more of a problem and the running costs get higher over time. 66s would seem the obvious successors as there doesn't seem to be anything else being introduced.
Basic economics works against small fleets of older vehicles - true of both road and rail - as they tend to become less reliable as they get older.
With the Networker DMUs - these are technically non standard, when compared with Pacers / Sprinters. They're wider, as other posters have pointed out which may prevent their use elsewhere without a load of infrastructure changes. Additionally, because they're non-standard, you have all the familiarity, spares and maintenance issues which go with non-standard small fleets. At least Chiltern know what they're doing with them, which is why they are the most likely home.
Either way these will only become available if London - Reading / Oxford / Newbury were electrified - and that's not likely in the next couple of years.
With regard to the PEP derived EMUs:
314s replacement seems to be happening in the next couple of years with new deliveries to FSR.
313s - London Overground is replacing their's this year - whether all will be taken on by FCC remains to be seen. Also FCC likely to keep theirs a bit longer.
507 / 508s - Merseyrail are likely to replace theirs in the next couple of years and it's unlikely any of the southern operators will want them hence likely to be scrapped.
315s - depends on the strategy for NXEA I guess.
317s - Unlikely to be suitable for FCC GN when linked to Thameslink, so unless they get moved onto NXEA, it's difficult to see where these would go as all other areas with 25kv will have newer units.
455s - I'd assume both SWT and Southern will be wanting to replace these with Desiros (SWT) and Electrostars (Southern) to standardise their fleets. Again once these operators finish with them, there's nowhere for them to go.
This is why I reckoned the PEP EMUs will be heading for scrap before Pacers / Sprinters and why the Networker DMUs also have an uncertain future.
But yes, there are some demic 58s lying around, which will very likely get scrapped in the not so distant future, although the same can be said for some 60s- there are a few of them that would cost a pretty penny to repair. There are probably more 58s in 'working order' than there are 60s thanks to the recent TGV Est work- so I can quite imagine in the short term demic 60s going before the 58s. 'Green Machine' 60081 springs to mind as being absolutely knackered.
