The 50:30:20 budget is a good place to start. The idea is that you spend 50% of your net income on needs, 30% on wants, and save 20%.
Needs are things like rent/mortgage, council tax, utilities and other bills, transport, insurance, basic/essential food/toiletries/cleaning products, childcare. Ideally, this should be no more than 50% of your net income.
Wants are things like clothes (although I appreciate clothes are essentials/needs, especially with DC, but keep it in this category for the sake of the model), luxury/branded food, treats and snacks, gifts, entertainment, holidays, gym/club membership and hobbies, subscription services like Netflix etc, pet care. If you are spendng more than 30% of your budget on this, you should try to cut down.
That leaves you 20% to save.
Realistically, if you have an income £2740 and are receiving UC, I would assume that means you are living in an area with high rental costs and a large amount of your UC is the housing element? I doubt you will be able to keep your needs at 50%.
It's a good place to start though. Set your budget out and check to see if you can make any savings on your needs section first eg can you swap branded foods/products for basic versions, move credit card debt to a 0% interest card, get a cheaper car, cycle instead of drive, get a better phone/broadband/insurance/energy deal etc? If you can't get it below 50% (I suspect not if you are on UC), split what is left of your budget as 60% wants and 40% savings eg if your needs can't be cut back to less than 65% of your net income, budget 21% for wants, 14% for savings.
Next, go through your budget for wants and see if there any cutbacks that you can make eg buy secondhand clothes instead of new, cancel subscriptions like Netflix, cut down on treat foods/snacks/fizzy drinks etc, take up running, walking or cycling instead of going to the gym. Try to at least keep this budget below the calculated percentage (if you need to spend more than 50% on needs) and definitely below 30%.
Obviously, once your savings reach a certain point, you will start to lose some UC. However, if it gets to that point, you can use some of the savings allocation to put you in a good financial position when your DC leave home and you are no longer eligible for UC or it reduces eg pay off debt, put extra money in your pension, buy a newer car that will last longer, buy items (eg furniture) that will last a lifetime (so you don't have to replace them when your income drops) or will appreciate in value, put money in a savings account in your DC's name for university (if they might go to university), or even go on holiday (why not, if you've been scrimping to save?). Before anyone shouts "deprivation of assets", no UC will not consider it to be deprivation of assets if OP is putting £25 a month into a university savings account for each of her DC or spends a few grand on upgrading a car or a holiday.