Firstly, do you live In England. If so, does your boss pay full-timers for Public Holidays? If full-timers are being paid for Public Holidays but don't work them, then you are also entitled to the same, pro-rata - by law. Based on the fact that a Public Holiday normally equates to one fifth of a full-timers working week, the minimum a boss can give you is one fifth of your contract hours as time off whenever a Public Holiday falls. So, if you work 20 hours per week from Tuesday to Friday and the PH is on the Monday, you should get 4 hours. Alternatively, you could be paid your full weekly wage, work your usual hours but receive an extra 4 hours pay. Companies employing staff who work VSA's often do things differently but this is often to the advantage of the staff involved and some receive more time off than others - depending on whether they were rostered to work on the Monday and if they're still required because it's business as usual. However, if staff are being paid enhanced rates for working on a PH but you aren't, it could come down to individual contracts and what you agreed your hourly/enhanced rates would be - your boss would not be acting illegally in this circumstance - just in the same way that some supermarkets pay long service staff double time but new recruits single time only.
This doesn't apply to Scotland and I'm unsure about Wales. There was a recent test case in Scotland that went through an Industrial Tribunal and the employee lost the case so hard luck if you live in Scotland I'm afraid.