No one will actually see what you’re buying every month, lenders will just want to see that someone has trusted you with a credit card limit, and you don’t abuse that trust by spending well within it, and managing your debt effectively.
Just get a credit card and put a tank’s worth of petrol (or all your petrol every month) through it. Set a direct debit up to always pay the full amount, and as long as you only buy fuel on the card, you can tell Monzo to classify that payment as fuel.
I’d disagree with @jzw95 actually, you should wait until you get your credit card statement before paying the amount off. It’s only at this point that your spend amount is given to the Credit Reference Agencies, which is what will build your credit history other lenders will see (ie. they’ll see you spending, say, £150 a month and being responsible by paying it off too) and it’s this behaviour which ‘builds’ your credit history for the future. If you spend on a credit card and immediately pay it off, when your monthly statement is generated it’ll apparently show a zero balance, which won’t have the ‘credit building’ effect you desire.
Building a good credit history is important, and those who’ve never even had an overdraft have demonstrably had difficulties when they’ve come to apply for a mortgage, as lenders have no history upon which to base their assessment of how you manage credit.
Couple of tips for success:
Don’t spend right up to your credit limit.
Always pay on time, and lenders will look more favourably if you always pay more than the minimum amount.
Don’t get carried away and think you have a load of free money to spend, you’ve always got to pay it back! Only use the card on something you’d be paying for from your current account anyway.
And finally, ignore completely the score the Credit Reference Agencies will show you. They make it up, and no one else ever sees it.