Write down everything you spend. Every last little thing. Don't try to change your spending habits, just spend as normal and note it all down.
Then at the end of the month, divide everything into categories.
Mortgage, council tax, utility bills, transport, food, credit card payments, clothes, socialising, coffee, phone contract.
Do you have a gym membership? Netflix subscription? Note it all down.
If you think a lot of it is going on food, divide food into subcategories. Ingredients to cook from scratch, ready meals, takeaways, work lunches.
You'll soon figure out where it's going and where you can cut back.
Do you spend on your credit card or are your monthly payments going towards paying off an existing debt? If you're spending on your credit card you need to note all of that down too. If you're just paying off an existing debt, can you pay it off faster? For example, when deciding whether to get a takeaway, could you perhaps have some pasta and sauce and chuck 14 quid at your credit card balance instead? Or in the morning, if you'd usually buy a coffee on your way to work, can you go without and pay off 3 quid towards your credit card balance? It'll soon add up.
If you have a phone contract, can you keep using the same phone when it ends and move on to a cheaper SIM only contract? And when you need to get a new phone, can you buy a handset outright using an interest free credit card if need be and get a SIM only contract with it? It'll be cheaper overall. If you have an iPhone, ditch it. You can get perfectly good Xiaomi and even Samsung phones for a couple of hundred quid.