The current legal entitlement is calculated as 4 times the number of days worked each week. So that gives someone, who works a 5 day week, 20 days paid holiday per year. That entitlement includes any days fixed by the employer (e.g. when the firm is closed on public holidays).
The Labour party manifesto, at the last election, included a commitment to increase the annual entitlement to effectively add on the 8 public holidays to the existing entitlement. (This is not the same as saying that anyone will be entitled to take a day off, or to receive extra pay for working, on a public holiday. As far as the law is concerned, both under the existing rules and under the proposals for change, a public holiday is just like any other day. If you normally work on Mondays, and the firm is open on a Bank Holiday, your employer can insist that you work at your normal rate of pay).
While the Government have not yet honoured their manifesto commitment, they have put forward proposals for the new regulations. The plan is that each employee will be entitled to 5.6 times the number of days worked each week. So someone working 5 days per week will then be entitled to receive 28 days paid holiday per year. (It's also part of the proposals that the statutory holiday entitlement will be capped at 28 days. So the '5.6 times' rule won't apply to someone who works 6 days per week).
Chris