Planning a train journey in the UK using any of the major booking services allows you to specify stations to avoid or travel via. I would like to avoid taking the tube. In this case it is only a preference because of luggage and a small person; for someone who couldn't manage stairs it would be much more significant.
Avoiding the London mainline stations doesn't do the trick: If I tell (e.g.) thetrainline.com to avoid Paddington on a route from say Bristol to Norwich it says I should change at Reading, take another train, then the tube.
If I specify a plausible alternative route (via Birmingham New Street), I'm either ignored or I get a ridiculous route that manges to include London and Birmingham. Doing this manually I can come up with a very slow journey, but I can't think of all possible interchanges.
So is there a tool that allows train routes to be calculated avoiding the tube? Or step free?