You want to see an ageing train fleet, head to Bristol and the south west. Barring the Voyagers on XC diagrams the last time a new train was seen west of Taunton was the Class 158 on selected Cardiff-Paignton/Penzance services. We don't even have them anymore!
The last time I was down there most of the units I were travelling on were class 158's.
Westbury, a 158 took me there.
Weston Super Mare, a 158 took me there.
Trowbridge, a 158 took me there.
Bath, a 158 took me there.
Salisbury, a 158 took me there.
Bristol, a 158 took me there (so that I could change).
Elsewhere in the South West it was either a 150 or a 143.
You do have one new batch of units though, these being the 387's although how closer they'll get to Temple Meads pending the electrification, I don't know. The next batch of 'new' DMUs though will be the Turbos as per the new toy that is running on the Severn Beach line.