Three regimes, three different ways of counting. This works out your deadline in working days, skipping weekends and England and Wales bank holidays.
This is an indication, not advice. It applies the counting conventions as I understand them and covers England and Wales bank holidays only. Notices can state longer periods than the statutory minimum, dates of knowledge are frequently arguable, and getting any of this wrong is unrecoverable. Check your own notice and take advice before relying on a date.
If the answer here is uncomfortably close, ring me on 07946 361275 rather than emailing.