A good visual guide from the Guardian
https://www.theguardian.com/uk-news/ng-interactive/2025/mar/24/visual-analysis-how-the-markets-boxed-in-rachel-reeves
But basically, the UK has had to rely on borrowing money as it spends more than it brings in.
It has borrowed money at low interest rates - but these rates have increased.
£105 billion on servicing debt interest. When you are borrowing a lot of money, even a small change in the interest level will massively increase the actual amount of money we need to pay on interest
Some context from the OBR on the budget
https://obr.uk/forecasts-in-depth/brief-guides-and-explainers/public-finances/
Income: £1149 billlion
Spending: £1276 billion
Of which £104 billion is on interest payments
Deficit: £127 billion
We need either more income, less spending and reduced interest payments.