You need a gas licensed plumber to close off the gas cooker, a sparkie to connect the new cooker and joiner to do your cabinetry. Imagine your insurance company not paying on a house fire because the gas leaked and the cooker sparked

If I did this to any of my customers who just want me to tell them how to get something done, they would quite reasonably have me replaced.

That’s not the problem. He won’t quote the whole thing, he just selectively quotes it and then says I have to have more people because he hasn’t done it properly.