Well thank you all for an overwhelming response to my question.
I feel honoured
I think the answer, as suggested by Mr Millard, is to use a smaller engine, perhaps an electric one.
This would allow cruising at a sedate pace of 4mph, which is the speed limit for most rivers and canals, and they only seem to cost around £100 on ebay I think.
It would allow an average Thames lock keeper to retain his nerves (on more than one occasion I've been taken to task for crusing past Mapledurham House on the Thames at 18mph (apparently the wake from the boat drowns little bank-inhabiting chicks))
My current plan is to travel the Islington tunnel in east London's Regent's canal. Spooky!