They didn’t have £1.4B of assets, they had £1.4B of equity (which is even better, because it’s already assets minus liabilities) but the catch is that the number refers to before the bank run.

It’s difficult to know how much equity was left after the bank run. Definitely less than £1.4B, but definitely more than £1. HSBC was able to buy it for £1 because SVB’s reputation was damaged beyond repair, so eventually their equity would have gone to zero without a bailout, hence £1 was a kind of decent bid considering the likely value of equity in the future.