The only place I could suggest for Mk4s would be to put them on the Kings Cross - Bradford/ Sunderland GC services to replace the "temperamental" 180s - but then this would create threads where people try to dream up a home for the (more modern - twenty first century) Adelantes!
Neither of those are under the wires all the way though, and a 67 wouldn't be adequate for the long, fast stretches of ECML.