I understand the thought process behind these kind of challenges, but honestly its kind of pointless-yes, you probably could spend £30 for one month on a very poor diet. That is completely different to being able to spend £30/month every month on food. It would be very unhealthy and I imagine exhausting. If you are wanting to discipline yourself and be more frugal in a way that is sustainable over time; whilst also eating a decent diet, £30 a week would be more realistic. This would definitely not include eating out or takeaways.
I consider myself careful with the food budget out of necessity; but part of the trick is to be able to eat enjoyable, nutritious and tasty food on the budget.
If you are not fortunate enough to have lots of spare cash each month for activities/going out (we are not-we have enough to pay bills/food/essentials and that is pretty much it), food becomes pretty important as a social activity in the family as well, so I want everyone to enjoy it! I save money on cheaper versions if it is the same product-for example, I will buy just essentials cheese rather than pilgrims or cathedral city; but coffee has to be a certain brand (tried the cheaper version, hated it!)
I'm just putting some perspective in these "challenges"-the reality of being on a tight budget doesn't mean everyone is sat joylessly over rice and beans every night-so I always find these extreme frugality challenges baffling.
If you are wanting to try some cheap recipes OP, making in bulk and freezing saves a bit of money-for example
Homemade chilli
Corned beef hash
Homemade soups (tomato, celery, veg, chicken, Pea and Ham)
Sausage casserole
Bolognaise
With mince based dishes, I use grated carrot, spinach and sometimes pearl barley as well to bulk out and add flavour. I have seen some add lentils but I am not keen on them.
If you are cooking a whole chicken, remove the legs and bone them, then stuff them-I cook the breast on the crown, I find there is less wastage this way.
It is also worth checking your local market as this can sometimes work out cheaper and for fruit and veg particularly, I have found the quality better too.