I always think self builds are so interesting because people build to suit their own lifestyle. So there's the 60s priorities: living room and dining room are one room then there's the hatch to the small kitchen but a good sized laundry room so there's inside hanging space. Bathroom to bedroom ratio is sensible and both bathrooms have the high windows so you have privacy but can still air the room by opening the window. Looks like there is a warm air heating system so no radiators using up wall space. Even the garden is in period.
It would be very easy to modernise looks wise but you'd probably need to completely rewire and add a ton of insulation and update the heating system. Layout wise stick a wall between the living room and dining room, and remove the wall between kitchen and dining room. Put in an Uncommon Projects kitchen, lux up the bathrooms to 21st century requirements, keep the beautiful MCM wooden shelving systems in the sitting room and study, update the soft furnishings and add some colour.