The UK’s largest bank is The Hong Kong and Shanghai Banking Corporation
The Q3 annual results make interesting reading:-
All reporting – Group | HSBC Holdings plc

Courtesy of HSBC Holdings PLC
The Balance Sheet reports total asset of US$ 3,324,223 Million = £2,425,518 Million
£2,425,518 Million = £2425 Billion
£2425 Billion = £2.425 TRILLION
The HSBC balance sheet of £2.425 Trillion is larger than the annual UK GDP