1) YOU NEED TO BUDGET - Pen and paper or an app, it doesn't matter, and write down all your ingoings/outgoings. It is the only way to track your spending and work out where you are wasting money, or where you could make more. If you want personalised advice post it here or on Reddit UK Finance.
2) Cut spending - most people don't have a "saving problem" they have a
spending problem. Expensive contract phone, check, expensive unnecessary "diet" food, check, regular hair and beauty visits, check, etc, etc, etc. You can look good inexpensively, you only have to check out the countless resources online to find out how.
3) Maximise income - work harder, for longer or smarter. Look for better paid jobs, work overtime, see more customers at work, sell anything you don't need, etc.
4) Minimise debt, particularly high interest debt (ie; overdrafts, credit cards, store cards, loans, car loans, etc) - interest and overdraft fees cost most people £100s a year. Getting out of your overdraft (if you're in it) and paying off any high interest store cards (or doing a 0% balance transfer) should be a priority.
5) Save on payday - set up a standing order and pay what you can afford (this is where your budget comes in again) into a high interest savings account. When it's out of your current account you're less likely to spend it unnecessarily.
***Sure switching bank accounts, cashback websites/credit cards/account-rewards, price comparison websites, etc help, but the savings are relatively meagre and the priority should be earning more, cutting spending and reducing high interest debt. ***You're on MAT leave (so I assume a pretty poor rate of pay) and yet you're spending £100s on a new phone, plus £10s a month in contract fees to "save money"? Your concept of saving is way off. A new phone is nice, but you're not in the position to splash £100s on a new phone and contract.
You could have got a
sim free contract for less than £10/month and simply continued to use your old phone at no extra cost, or got a cheap second hand handset (if necessary).
Ditto spending money on "special" weight loss food. They will make little or no difference to your weight loss. You just need to eat slightly less (ie; cut out crap/fizzy drinks/snacks/deserts, reduce portion sizes) and eat slightly better (ie; homemade soups, frozen vegetables/chicken/salmon, etc) which if anything will probably cost you LESS.
You have a spending problem, not a saving problem.