In those cities you are likely to see plenty of 60s & 66s (possibly a 59?). EWS stable locos next to Newport station & the occasional shunter can be seen passing through the station. 37s at Cardiff of course, plus the usual freight, 60s & 66s.
The EWS side has shut, or at least it has last I heard, I dont think its reopened.
Also, Look out for railtours on those dates, might be something good.
good photo locations, ones i know are, are the london end of the Newport River Bridge, Alot of good fots been taken from there. there is also the other side of the tunnel out of newport (not the station end, the other) where shots have been taken of trains leaving the tunnel. and, of course, there is newport and cardiff stations them selves, along with swindon.
Swansea, you will see eff all. HSTs and bog units. Nowt else. Unless the Serco Test Train comes in, or the NMT.
Cardiff, 60s and 66s on freight and light engine moves. Rare 37 freight (it's happened (unless it came from Tidal) recently, with 37405!) and slightly more often 37 light engine moves. HSTs and 37s on Rhymneys the only other interesting activity. 59s through Cardiff I'm not sure about, I know they run through Newport rarely.
Newport, 60s and 66s on freight, lots of light engine moves. 09s come out a lot too, light engine and on freight moves. 57s come out in the morning and evening, times I couldn't be more accurate on, but I know those are the periods of time they're out. FL 66s also come up and down in the afternoon and evening, sometimes morning, but again, can't be more accurate re: times. Newport is the freight capital of South Wales, spend a couple of hours there and you'll be sorted. Godfrey Road (the stabling point) is NOT busy during 1200 - 1430. Early to late evening, it most certainly is. ADJ is down the road from Newport, heading WB to Cardiff. Wagons and locos galore. East Usk (another yard) can be seen heading EB towards Severn Tunnel Junction and England. Again, a good few locos and tons of wagons. Best advice to get Freightmaster, check the times of stuff in there, as well as Rail-Gen, to get the best times to go here.