With reference to The Clansman, my memories of it are at odds with what's been said, but I suspect changes over the years have made both of us correct.
Firstly, the northbound London - Birmingham/Wolverhampton section was NOT scheduled to be part of the half-hourly London - Birmingham/Wolverhampton express service. Going by the May '74 - May '75 PTT (of which I have a copy), West Midland expresses left Euston at XX10 and XX40 - the Clansman left at 0935. The full M-F timetable is shown below (note that the tabs don't work, AND note the stations where they uplift only).
EUSTON (d) 0935
COVENTRY (u) 1039
BIRMINGHAM NS (u) 1105
WOLVERHAMPTON (u) 1124
CREWE (a) 1203
CREWE (d) 1206
PRESTON (a) 1243
PRESTON (d) 1245
CARLISLE (a) 1354
CARLISLE (d) 1403
MOTHERWELL (a) 1518
COATBRIDGE CENT (a) 1538
STIRLING (a) 1605
PERTH (a) 1649
AVIEMORE (a) 1850
INVERNESS (a) 1945
I THINK the change of loco was done at Carlisle - 20 minutes between Motherwell and Coatbridge seems a very short time.
Having said that, the SOUTHBOUND Clansman arrived at Wolverhampton at 1905 and from there worked to Euston as a scheduled West Mids. - Euston service.
I don't know where I got the info from, but when HST125's were introduced, they started the Kings Cross - Inverness service. As a result, the traffic on the Clansman fell away, so they re-routed it via Edinburgh. This didn't work either, so they scrapped the Edinburgh - Inverness portion (removing the 'Clansman' train name), which made things even worse. I can personnally attest to this, having once travelled on the Carlisle - Edinburgh leg of this service. My wife and I were the ONLY passengers on the coach. Eventually, the entire service was done away with. In my view, there is a case to re-introduce a Birmingham - Perth service (via Cumbernauld) with extensions to Inverness and Aberdeen.