You could reinstate stud walls which entails a great deal of expense regarding floors, cost of stud walls, doors, making good, related electrics etc. I'm not sure why you want to do this? Could you get away with internal glass walls or separation doors? You could also use tall bookcases to create partitions. You could use the family room as a dining room and then close off the lounge & current dining room with 2 sets of doors.

The plan doesnt show the lounge and dining room as having any windows, is that accurate?

Agree with Isla. Suggest you live with it for a while before making any changes. Do you have a large family of differing ages? What is driving you to wish for less open plan?

My partner is a general builder so adding stud walls is relatively easy / cheap! The space just feels too open and not cosy. We want to have a separate living room. We have two children so creating separate spaces is definitely needed! And there are two windows at the front in the lounge, and then the kitchen area has two sets of sliding patio doors :)

Some ideas: Moved front door, created a long hallway full of storage and utility options (full wall of bespoke units and integrated bench under the window). All glazed doors to let in ample light for that open feel (frosted is fine), not in scetch an optional side entry. Former entry becomes shower room or wc. Closed off the lounge. Kitchen layout added just for fun, i imagine looking out onto the dining and into the garden would be lovely. Multifunctional room on the top left gets some privacy from room dividing panels. And congratulations with the new house!