What is the penalty for filing UK accounts late?

It depends on how late they are: £150 up to 1 month, £375 up to 3 months, £750 up to 6 months and £1,500 after that. The penalty doubles if the accounts are late 2 years in a row. It is charged to the company automatically.

Accounting and filing support

Source: Late filing penalties, Companies House, GOV.UK