I think Virgin E and W Coast are always at the London end
Crosscountry / EMT is never guaranteed as trains may frequently reverse during their route then go into and come out of the depot in any direction.
Platform displays and announcements will usually tell you where 1st class is - but usually say it is at the FRONT or REAR and if you aren't familiar with the station, you might not know which direction the train will arrive from!
Raffles is correct about weekdays, but on weekends, Monday mornings and Friday evenings, 1st class is usually full anyway, so if taking the train from its origin you just need to arrive early.