Might help if I added the floorplan!

The simplest solution seems to be to leave the utility as is, and move the kitchen to the conservatory. There seems to be plenty of space to create an office and still have a nice living area off the dining room. One potential drawback here might be the position of the utility door, off the living space... I don't know if that would be an issue?

I think you have to be careful to ensure you end up with better spaces than you start with so would be reluctant to put the kitchen in the small conservatory.

However with your budget you can consider knocking the spaces together and rebuilding the conservatory. Money will be tight and you might have to cut the spec of a few things.

One possible economy is buying a used kitchen. There are a couple of great sites found quickly with a search. There are some great kitchens bargains to be had.

And this is the plan

That's the sort of thing I had in mind! my husband likes it too. Sorting the conservatory shouldn't be too much of a problem, it is an extension with plastic roof which needs to be changed to glass and there are already skylights in the utility room.

Thanks for the plan, it really helps.