There are two types of holiday entitlement. The basic minimum entitlement set down in law is called statutory holiday entitlement. Your contract of employment can provide for more generous holiday entitlements than statutory holiday entitlements but it cannot give you less.
If you work full-time set days then your statutory holiday entitlement is 5.6 weeks of your normal days. This is subject to a cap of 28 days. The law does not distinguish between normal days and bank holidays. There are normally 8 bank holidays so if you are full-time and your contract provides for 20 days plus bank holidays this will be the statutory minimum.
Part-time workers have a prorated holiday entitlement of 5.6 weeks of their normal week. This can make the calculation complicated. Someone on three days will receive 5.6 x 3 giving them 16.8 days holiday which is usually rounded up to 17 days. There are additional rules about the first year of employment. Holiday pay can also be complicated so you should seek advice if you are an employer who is unclear about what to pay employees and as an employee if you think you are being incorrectly paid.