1S51 is Birmingham and Leeds (I always get off at York) but likely Newcastle? as well.
It's either Newcastle or Edinburgh (or slight possibility of both). Edinburgh XC crew sign Leeds - Glasgow but Leeds-Glasgow-Edinburgh in one go (as it would have to be) is probably a bit much; considering they'd have to come down from Edinburgh as well.
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WCML Holyhead does depend. Only Holyhead TMs sign beyond Saltney Junction so they have to have got on the train at some point. Sometimes that point is Crewe; sometimes Chester; or sometimes Euston (they sign all the way). The Wrexham service is a Liverpool driver and TM
As has been said; for drivers I think it's Manchester and/or Liverpool that sign to Holyhead and they generally change over at Crewe I believe.
For the Chester terminators I think it's Euston; Liverpool & Preston; so they all do it at some point. Changeover point is most likely Crewe; possibly Chester. It's fairly unlikely I would say that Virgin crews change at MKC because if there is ever disruption there will be a lot of trains sitting on the fast with no crews. London Midland do change there occasionally but that's by the by
ECML Edinburgh almost always change drivers at Newcastle. Guards is a bit more complex as Edinburgh guards sign down to York I believe; whereas drivers only go to Newcastle. Drivers occasionally change over at Doncaster as well
ECML Aberdeen - there are top link Newcastle drivers much like Inverness; they drive Newcastle - Edinburgh; PNB; Edinburgh - Aberdeen; shunt at Clayhills for a bit; hotel; shunt off Clayhills; Aberdeen-Edinburgh; PNB: Edinburgh-Newcastle. Edinburgh drivers have one out and back job there a day (leaving Aberdeen at 1818). Inverness is done by Edinburgh guards (who work the morning train back the next day; they don't do the Aberdeen shuffle) ; the out and back Aberdeen is also done by Edinburgh guards; all other Aberdeen services are done (with commendable simplicity) by Aberdeen guards.
WCML Liverpool/Manchester are I think generally out and back turns but not all; some are different. Birmingham services are a bit more complicated I believe.
The only service between two major cities I can think of that doesn't routinely change crews
en route somewhere at some point is Edinburgh-Glasgow.