I've been reading up on the restoration of the Cotswold canals, which when completed would link the upper Thames to Gloucester and the Severn / Bristol channel. For me it would provide and almost perfect circular route. Down to Brentford on the GU, up the Thames to Lechlade, down to Gloucester via Stroud, up the Severn to Tewkesbury, then onto the Avon through Evesham (near where I was brung up), to Stratford, then up to the Midlands to pick up the GU for the journey home.
The Cotswold Canals Trust has been doing sterling work in pursuing the restoration and are well on there way on this very ambitious project. It might well be another ten years before the trip becomes a reality, but if we still have a boat and are fit, I'd love to give it a go.