I would suggest gravelling the area you no longer want. You will need to skim the top layer of grass off first, put down some good weedkiller, cover with weed fabric (we use the black stuff) and cover that with gravel/stones/pebbles....hope my suggestion helps?

if needs be concrete .
Get a friend or relative to take the lawn up , put hardcore down and cover with concrete.
Doing it this way you can have large pots with beautiful flowers herbs or veg in
them

You can put pots on gravel too :o)
I think it ruins a garden when you simply concrete it, especially since there are so many different colours/types of gravel/cobbles available.

It's really difficult to separate fallen leaves from gravel when autumn comes, paving slabs are easier to sweep clear and have more texture and variation available than plain concrete, so I'd go for slabs. They're available in all prices from budget to luxury and are much less work than grass.

what about decking

if you went for gravel,,. if finances allow you could make apart of it block paved for a ,bench,seat and perhaps raised beds at waist level