Here's a tour that I have discussed before in other places. It can include various routes which are these.

Edinburgh to London (the old A1)

York to London (via Doncaster)

York to London (via Lincoln roman road which is Ermine Street)

There's also 2 options when getting to Alconbury to go via Stevenage if your doing Great North Roador Huntingdon and Warehouse if your doing Ermine Street. I think that a route like this would be the UK's answer to Route 66 in America.