Customers have already agreed their tariff (how much they pay) with the supplier, since a massive spike like this wasn't predicted and the supplier is obligated to keep paying to supply their customers regardless of whether its at a loss some suppliers with limited capital reserves are quickly burning through them.

There's also a price cap for how much suppliers are legally allowed to sell their energy for in the UK (set by ofgem). If the price of energy exceeds this value, every supplier will be selling energy at a loss till ofgem increases the cap.

To get around this, most suppliers purchase their energy a few months in advance, but the ones who haven't will be in trouble currently.