For
158s the EMR stock rostered shows on Real Time Trains the night before and 158s are pretty regular on the Norwich, Peterborough - Nottingham - Sheffield - Manchester - Liverpool turns (but 170s are also used now for some trains on that route so this is worth checking)
It doesn't always work but what I do is look at what happened say yesterday (but perhaps not a Sunday but you get the idea) on RTT to see what might happen on the exact same day a week later in terms of what rolling stock is allocated. Doesn't always work but it gives you an idea for planning
For
Inter City 125s if funds permit a trip to Scotland would afford a better travelling experience because the Great Western HST sets have had their interiors made so unpleasant IMHO (too cramped, almost all uni directional airline seating which destroys the benefits of the widows and the group of 4 seats in the middle doesn't even line up with a window IIRC) in comparative terms.
Having said that the more close withdrawal dates for the GW HST sets means there is an argument for going to find them sooner rather than later after your exams.
For that you could maybe from London do SWR from Waterloo to Exeter on a
159 (maybe with a cheap Advance ticket to keep costs down there and back at start and end of the day - the earlier you go in the morning if very early likley to be cheaper tickets and you get more time in Cornwall) - then use a day rover or ranger (I think there is one?) for Devon and Cornwall to get from Exeter to Plymouth on any train permitted, then from there you would be with the GW HSTs for the day with unlimited rover ability to get on and off sets until time to get back to Exeter for a 159 home again.
Again the GW entries on RTT have the stock used listed from about midnight the night before which would help you find the HSTs when you get to the South West
The 125 Group are no longer doing diagrams but the info they do give is here and worth a look
www.125group.org.uk
For tickets using Rovers which gives you the flexibility on the day see here
How to find rail rover and railway ranger tickets on the national rail network. Rover tickets are cheap tickets for rail travel
www.railrover.org
For example I see for the South West you have a Devon Day Ranger and a Cornwall Day Ranger - £11.30 for Cornwall for the day with a railcard by looks of it
How to find rail rover and ranger tickets on the national rail network. Rover and ranger tickets are cheap tickets for rail travel
www.railrover.org
Ref 225s the advice above I would concur with - fairly reliably allocated to the services they are supposed to operate on so you could chance it with cheap advance tickets there and back.
My hunch would be that cheaper tickets are available in Advance before school holidays start and on mid week days (but probably not Fridays, sats and Sundays)
Hope this helps a bit
The TFW loco hauled Mk4 sets must be well worth seeking out - something I've not done myself yet
EDIT - for an HST 125 trip the 125 Group have this on sale with one of their sets at the moment which looks really good to me
www.125group.org.uk
This is only £35 for some interesting route mileage - if open to non members and looks like it is as it says 'the public' and I reckon you could get to Derby fairly cheaply on the Saturday morning from London and you are ending up in Reading so easy to get from there back to London which looks like where you are based.
SATURDAY 19 JULY – ONE-WAY TRANSIT TRIP FROM DERBY TO READING
In conjunction with the operation of the UK Railtours trip from Paddington to Pembroke and Tenby on Sunday 20th July, 125 Group is please to announce that the stock movement of the HST is being made available to the public as a one-way trip between Derby and Reading. Not only does that give the opportunity for folks from the Midlands and the South to ride our HST at a hugely discounted price, b
ut it is also operating by a rare and interesting route around the West Midlands.
Our planned route is:
DEPART: DERBY approx 14:30
