Thanks for all the input, guys.
So far I have managed to get on couple of these boats but all had either 25 or a 30hp. Only one guy went for the higher power but others just used what they had on their previous boat. The boats handled really well and dealing with locks much better given the overall stability and control compared to a SIB. I couldn't find a boat with a 15hp which I could try so far hence the confusion. These boats plane, at least going by some videos on Youtube but with a 40hp or a 60hp. The dealers are probably correct in advising a 15hp considering that these boats are largely used for lazying around on the Thames and visiting the various Boat festivals and Regattas but what my concern is whether even for this purpose, are 15hps just about enough or I can do with a higher power which will give a better/smoother cruise. Planing is not on my agenda due to speed restrictions and I don't think I will be thinking about the sea for at least another 4-5 years.
Pikey Dave's comment about Tidal Thames is valuable and I may have to ask the dealer to advise appropriately. I was intending to go for a 25hp but will seek further opinion.
Fenlander, as for the boat choice, you are correct- I was always keen on getting a Cruiser with added benefit of a cabin, toilet etc. I wanted something in the range of 20 feet (to not spend a lot on mooring fees) and had narrowed down on Shetland 4+2 and a Viking 20 but had to change my mind as soon as I stepped into one of them. The problem- my claustrophobia
Plus the social space on these cruisers around the helm is limited and shallow which could be a problem for our son.
A friend of mine who owns an Antaris Fifty5 recommended the Sloop and it fit in every bill. It is an open space with helm at the stern meaning I can keep an eye on the kid, reasonably deep to not worry about our son falling overboard accidentally, enough seating for just my family and dog or if we wanted to invite a few friends over; converts to a really spacious double bed to easily accommodate all 3 of us in case we ever feel adventurous and plan to moor overnight somewhere; comes with cruising canopy to deal with the elements and has steps to get out and in at Locks, lots of storage plus easy access to all areas of the boat. The Sloops do have a dry toilet but with pubs and restaurants littered across the Thames in our area, I don't think we will ever use it. Another option was a Bowrider but the Sloop faired better comparatively.
Buying a new Sloop was never my intention but used ones are rare and even if one becomes available, it sells in just a couple of hours for the asking price despite registering a very high interest with the Marinas. My interests are still in the used ones for all the reasons you mentioned in your initial reply but the dealers have a deadline to order new ones for next season which saves EUR3000 on transport/delivery. Guess that's business.
I still covet the cruiser and may buy a bigger one once the little one is a bit older.
If you have recommendations for another boat, please do tell.