They have their own Maintenance Depot, owned by Siemens, at Ardwick, Manchester, so it wouldn't be appropriate on the XC 170 routes or Waterloo routes as has been suggested. The most suitable would be to go to ATW, or whatever follows it in the new franchise in 2018, as they would be easily accessible to the North Wales Coast or the Marches. This has already been posted somewhere..
Siemens have a maintenance depot in Southampton (admittedly for EMU's), so the works involved in changing that to also cope with 185's would likely be cheap enough to justify moving them, especially if the lease is for a long enough period.
Anyway, given that SW wouldn't need all the 185's there is still scope for the Welsh franchise to gain some units.
Let's say that SW took 16 (enough to reform into 8*4 coach units and 8*2 coach units) that would be enough (with spares) to run the Yeovile to Waterloo half of the timetable (i.e. not the services to Exeter) as 10 coach units and the Salisbury 6 services as a 2+2 coach trains.
That would leave 5 units for Wales to have and could release 159's to release 158's, either direct to Wales and/or to another TOC to release other units to Wales (I.e. ones that could be useful in the Welsh Valleys).
How would they be used on the North Wales Coast services and what would that mean for the 175's and their depot(s)?
Just as an aside, 5*185's would enable the 15*175's that run the North Wales services to be run in pairs on 1/3 services. That could well mean that rather than running a 3 coach unit they run a pair of 2 coach units and use the three coach unit for services currently run by a 2 coach unit, which could mean an uplift in capacity over several services. The problem is, if the 185's are needed to replace 3 coach version of the 175's then even if you change them to all standard class seats your looking at about 188 seats (185) vs about 202 seats (175), so you'll be reliant on more 185's running in pairs to increase capacity. However, as the 185's are likely to be more expensive to run the franchise holder would be resistant to do so, a lot more so than running pairs of 175's (which could be 2*2 and still increase capacity over the 3 coach versions) which would be able to better match capacity requirements (2,3,4,5 or 6 coach long trains rather than 3 or 6 coach trains with the 185's).