It shouldn't cost anything like £15000 to remove a conservatory!
It's quite a simple job that could be done by 2 men in a couple of days, so it's just their labour cost and the price of a skip.
If the conservatory is of aluminium construction there will be a bit of scrap value too.
Are the builders you have contracted to build the extension not interested in demolishing the conservatory?
I have, in the past, taken down several conservatories prior to building extensions, but ,alas, I am now too old to take this on.