I don't know about the WCML, but for the ECML, it does vary depending on where the drivers are based.
Bounds Green - London to Leeds, and Edinburgh. A lot of the turns schedule a crew change at York or Newcastle for journeys to Scotland, but on the longer distance trips (Aberdeen and Inverness), it's usually Edinburgh where the crews swap over
Neville Hill - Leeds to Hull (for ECS moves), London, with a few crews passed for Doncaster/Leeds - Edinburgh
Edinburgh - All Scottish routes, and ECML to London, with a few crews passed for York -Leeds.