@jaybee £10K should be easily doable for the size you are looking at, couple of suggestions to keep the cost down, only "fancy clad" the sides you can see, so if this is at the end of a narrow the garden just make the front look nice and clad the back and sides in cheaper materials (definitely dont cedar clad the back for example if you'll never see it !) . Also, start looking out now for PIR on facebook market place, often get some very good deals on there people with a couple of boards left over, you'll have plenty in no time. I wouldn't skimp on the roof mind you, rubber seems to be the agreed best route there.