For what it's worth, on 1 August 1767, this notice appeared in the Oxford Journal:-
'It is proposed to make a navigable canal from the canal now making along the Vale of the Trent in Staffordshire, to pass through the parishes of Tamworth, Amington, Polesworth, Grindon, Atherstone, Caldecote, Weddington, Nuneaton, Chilver's Coton, Bedworth and Foreshill [Foleshill] to the city of Coventry, with communications to all the great collieries betwixt Tamworth and Coventry, and a survey has been made and found practicable and [?construction (word missing)] will be undertaken as soon as an Act of Parliament can be procured; And a further proposal has been made for continuing such navigable canal from the city of Coventry southwards through the parishes of Stoke, Binley, Newbold-Revel, Lawford, Rugby, Barby, Willoughby, Granborough, Stockton, Ladbrook, Wormleighton, Banbury and rom there down to the City of Oxford, to communicate with the navigation of the Thames.'
No mention whatever of its being a 'bottom' route, but perhaps that isn't entirely surprising at this date. What the notice strikingly shows, is that in their initial conception, the Coventry and Oxford Canals appear to have been viewed as a single waterway - a detail which may owe something to the energetic promotion of both by Sir Roger Newdigate of Arbury, sometime MP for Oxford University.