Cycling to Bristol from London

this is the route we used to use for TNRC into Bath (pre-two-tunnels) https://www.strava.com/activities/134490111 it dips a little lower but the route is mostly flat and takes in some beautiful countryside.

I would recommend your route into Bath through the two tunnels if you haven't done it before but the canal tow path is fine on a dry day and takes your straight in from Monkton.

The canal towpath is very narrow for some stretches between Devices and Bradford on Avon. Perfectly cyclable but you might end up negotiating pedestrians, dogs etc.