The original statutory holiday entitlement was 4 working weeks (i.e. 20 days for those working a five day week). Many employers chose to provide those 20 days in addition to the 8 public holidays but (since there was no obligation for them to do so) other employers only offered 20 days including the 8 public holidays.
After extensive campaigns by several of the large unions, the Government agreed to change the rules so that those people who had the public holidays included in their annual entitlement would, in future, have them in addition to those 20 days. The change has been phased in, meaning that those who work 5 days per week now get 28 days paid holiday per year.
The increase in holiday entitlement was only ever intended to ensure that those who previously got 20 days including public holidays now get 28 days including public holidays. No increase was intended for those who, like yourself, already receive 28 days paid holiday per year.
Chris