I think you meant to quote me.
Most international students are very rich - the sort that make blank checks out to their kids. The OP clearly isn't if she's even asking the question. I was one of the few, and if you try to keep up with the others no amount of money will be enough.
There are more of the 'poorer' types these days as the £ has weakened especially and costs of universities in other countries have increased. In mine for example we can do twinning programs spending a year or two in the U.K.
Catering type jobs are plenty easy to come by - especially in London! The best part is they are zero hours, you can pick and choose shifts as you want. It's also extremely easy to get, they take anybody with a pulse now that all the EU people have left.
Also, if you read my post properly I said £30 - £40.
From Tesco:
Laila Basmati rice 10 kg : £14.40 - 88 servings, working out to 15p a portion
Tesco Chicken Drumsticks 1.5 kg : £3.75 - 0.9375 per serving
Sauces, condiments etc - Shouldn't be more than 10 p per serving
Tesco Frozen Brocolli Florets £1.30 900g - 14 p per serving assuming 100g per serving
That's £1.50 a meal, multiplied by 14 comes to 21, And I haven't included breakfast :) You can get 1 kg of oats for under a pound, milk 1 p a litre. Alternatively go to bakeries at closing time you can get stuff half price.
You can even get things like Morrisons' Ready made chicken, at closing time they bag the stuff up and I got 10 drumsticks for £3. Apps like Too Good to Go, again leftovers for very low prices.
Of course OP is free to give her child more but she can always increase it. You may want to be more comfortable but to say £40 a week isn't enough ... hmm not quite true.