As i sail in the sea i have no idea about inland waterways, so roughly how much would it cost to keep a 30 odd ft boat in somewhere like Shepperton and what else is required such as river licence? etc
River licence, just over £500, mooring fees, diesel, insurance will give you a total of about £4.5k per annum. If you are prepared to moor bankside and can find a mooring that's not in a marina, you could run a 30ft boat for £2k to £3k per annum all in.
But you'll be saving on fuel costs over the winter because of not being able to use the boat due to either the river flooding or there are lock closures or both...