I am new here on this site and live in the USA (zone 5 which means it gets a lot colder here that what England appears to get based on a simple internet search - zone 6 to 9. MUCH WARMER), so my info is not British and I am likely violating protocol. Sorry. But I have 40+ yrs of experience gardening where I live and thought I may have a tiny amount to add to this conversation.
First, the other posts are great and should be used as great input.
Your post does not indicate if you hope to create an awesome vegetable, herb, flower or mixed garden. IMO the more mixed the better. Not only does variety add interest to the garden along with getting fresh produce, but it also is more resistant to pests and disease. You should read about companion gardening to find which plants work best together and which should never be planted anywhere near each other.
Next, the use of perennials will keep costs low over the long run and provide stability to a garden. For example, I have not purchased beans, peas, potatoes, onions, pumpkins, squash, tomatoes and some other crops for a decades. Many gardeners are used to perennial shrubs and flowers will be there for a lifetime, but do not understand the same is/should be true for vegetables too. My situation is different from yours, but you get the idea.
Next if you drink coffee you need to save the coffee grounds. If you eat eggs save them too. Put this gold in the soil. They are like catnip for worms. Worms are your friends. Make them happy and you will be happy. The requirements for a plant to be great are 1) the right type and variety of plant for the area (trying to grow oranges in Canada will never be successful), 2) the right amount of sun (trying to grow apples in a cave will not produce fruit), 3) the right amount of water (habenaro peppers are not nearly as good when grown with too much water), 4) the right food (fertilizer - both macro and micro nutrients are essential), 5) the right PH (strawberries do not like soil that is too base), 6) timing/temperature (plant tomatoes in January in Alaska, and it doesn't matter how many of the other things you do perfect, they will not grow), and 7) the right soil (carrots grow poorly in a rocky clay mixed soil). Worms help with several of these. If you do them ALL right, you will have a great garden. To have a very good garden only means you (like the rest of gardeners) is still learning to get them all perfect, but we still produce ever better results.
Next save your non-meat table scraps and put them in your garden. If it is waste and not meat based it belongs in the garden - period. It is FREE fertilizer and helps in other ways.
I am better than most at math, have saved detailed records and have done the calculations. Generally speaking I generally get 4 to 40 times the value (based on market prices) compared to what I spend on my garden each year. But I have a lot of experience. I also intentionally produce a lot of produce beyond my wife and my needs that we intentionally grow to give to the food bank.
We all have our unique situations. It comes down to getting a broad spectrum of input and experimenting to find the right formula to make our garden successful.
God bless and I wish you success on your gardening journey.
Just this old guy's 2 cents.