I know each of us has different circumstances....there's no way our monthly food budget could ever be around £20, I would struggle with £20 per week. We have a normal size garden so cannot keep any livestock, which means we have to buy meat and dairy stuff. We did briefly consider keeping chickens (we have the space for up to 6) but after working out pros and cons we discounted it - there were far more cons than pros.
We are growing as much produce as we can, however, this is only our 3rd year of doing it seriously and we are still on a learning curve, working out what works in our soil/climate/area and what doesn't, what we need more (or less) of, what isn't worth growing in future due to poor yield or too much work involved to grow it. We are aiming for eating seasonally but also growing enough storable or freezable produce to keep us going all year round, we'll get there eventually, I'm sure.
For the next few weeks I am going to monitor our shopping, seeing how I get on with writing (and keeping to) a shopping list each week and working out if we can go for longer than a week without shopping (better planning, in other words). I'll keep all receipts and in a few weeks hopefully I'll have a better idea of a monthly shopping figure.