Are you riding the tandem on your own? I wouldn't want to do that for that far - Virgin let you take tandems on the pendolino trains? It's how I got mine back from Manchester.
Fortunately not, the stoker and I are getting the train early early saturday morning, and hoping to ride it back in one day, looks to be about 120 miles..
So I'm cycling from Birmingham to London this coming weekend to collect a tandem. Any advice on the best route to take?