Wow what a wonderful plot you have OP. You need to start by observation. Make a plan with measurements on a big piece of paper and mark in the direction of the sun and orientation and any shade. What is the climate? Sunny mild wet cold dry? SW in UK is prevailing wind direction so record that.
Mark in any microclimates eg where does the wind swirl around, where is it dry or likely to be flooded, any frost pockets? This is vital at planning stage so you don’t waste time and money. This can guide your design. Then buy a cheap soil testing kit and measure the alkaline/acid levels of soil so you know what will thrive and what would struggle.
Have a big discussion with everyone in the household about what they would like from the garden so you can make a plan suitable for your individual needs egare there children who need places to play, can you have water or worried about little ones, pets, hobbies, space to practice sports (football cricket badminton basketball hoop etc?).
What is the house like? It’s good if the garden can complement the house eg if built of stone use the same stone to make garden walls or if terracotta brick use in paths.
Copy your paper then have fun doodling different designs. Use interesting shapes to break up the square and straight edges. I would have a huge pond with a waterfall as this is a wildlife magnet, lots of trees and winding paths and interesting hidden surprises to come upon.
Mark in where you want trees and think about where they will cast shade at different times of year. You need somewhere sunny for a sitting area and somewhere shady for hot days. It’s getting a bit late for tree planting so make a plan for the autumn as they are best planted in the dormant season.
What a great project - enjoy every minute.