Well it has been true most of the time in my experience- certainly in Brighton and Hove . Unless of course you find a buyer who wants to keep it . Most want to knock it down and factor that into their offer .
That’s good you’ve found a solution.
You would have added real value by knocking it down and incorporating that extra space into your kitchen extension .
Conservatories are viewed by most buyers as very old lady ( I think that’s the phrase in England ) and ugly .
I did unfortunately have to devalue it because although we had about 20 viewings from memory ( it was a while ago ) lots of people remarked about the cost of knocking it down and what it would cost. No one wanted to keep it . The eventual buyer brought round a builder on the 2nd viewing to get...
Houses with conservatories are usually worth less than ones without ! Very 1980’s which isn’t desirable nowadays . Obviously a proper extension is the best way to add significant value .