Agree that Ebay, Preloved and local FB selling groups are your friends. You just have to be careful, check everything is in excellent condition. Don't be afraid to say no if its not pristine.
I have used Ebay for solid oak furniture (console table, chest, side tables) and if you put some time in, you can usually find local suppliers that sell end of lines / returns (but brand new) on Ebay for sofas / armchairs. We bought our M&S sofa bed from an Ebay supplier, perfect, brand new, for about a third of the RRP in store.
Ask for recommendations for a curtain / blind maker locally, get fabric in the sales / cheap online (again, Ebay is very good for Sanderson / Harlequin / Designers Guild fabric that might be the end of the roll etc) and have it made into curtains, blinds, cushions etc again much more cheaply than you'd pay for ready made items.
I have also had some bargains from the local FB group when people have just got fed up with items / moved house etc and for things like ottomans, picture frames, mirrors, shelves, its great. People often get rid of items too when their children grow - have bought children's bed frames (would always but new mattresses) from local FB group (and once bought 2 matching Laura Ashley white wooden bed frames from a local lady after seeing an advert in the supermarket!).
TK Maxx and Home Sense are good for bedding / lighting / kitchen utensils. Ikea great too for all those practical bits. The large supermarkets (Sainsburys) often do good deals on their home ranges and certainly at Sainsburys, they had events a couple of times a year where your nectar points can be doubled up - we bought a TV from there relatively cheaply by using points off 2 different Nectar cards last time they did that.
Wallpaper / paint - some of the brands / manufacturers have factory shops so worth googling to see what you have locally.
And yes, invest in a bed and good quality mattress 