I'm based, like you, in the UK. I've taken 10 river cruises, all with Viking, and no ocean cruises. But from reading this board I think you'll find that river cruising is nothing like ocean cruising except that you're on a boat. The size of that boat is governed by the size of the locks ab height of the bridges on the river.
So you may find the cabin smaller and a lack of entertainment found on ocean cruisers,. But every day there is land based entertainment. When the boat is moored you're free to just walk off and explore. No tenders.
As you rightly say, there are no 'sea days', every day is a see day - i.e. there's something to see on both sides of the boat and you're never out of sight of the land.
You don't say where in the UK you are, but if one of the major airports is near you then you are only a short flight to/from the start/end point.
The river cruise companies like to sell you a hotel based land extension at either end. I don't think they are good value, as you are not far from wherever they suggest you can make your own visit there anytime.
I like to arrive at the airport, be taken straight to the boat and unpack once.
There are a tremendous number of river boat companies, so you have a wide choice of costs and offerings.
On travel agents - you'd have to find a specialist in river cruising - high street ones are no use. I have always booked direct with Viking.
Most people on this board are in the USA and the offerings and terms there differ from the UK - they seems to be less advantageous, so take note where a poster is if they're talking about booking terms.
If you are willing to book at short notice and flexible with dates there are often bargains to be had, so its worth getting on mailing lists of companies that interest you by asking for their brochure.
Viking operate about 70 river boats, and market different ones to the USA, Australia nd the UK. IF a boat is not fully booked they open up bookings to other markets at a discount. I've gone several times on trips on dates not listed in the UK brochure/website; I don't know whether other companies operate like this but if you don't mind being one of the few Brits among Americans and Canadians it's worth calling the company and see if they have a sailing coming up soon.
I wrote a review of the Rhine Getaway trip we took in April - it's here
https://www.cruisecritic.co.uk/memberreviews/memberreview.cfm?EntryID=687006