(2-part post):
Subject to giving reasonable notice, an employer is completely free to determine when an employee's holidays should be taken. So you can insist that any particular dates that you choose (including Christmas Day or any other dates) must be taken as part of your employee's holiday entitlement.
'Bank holidays' are only defined by law for those who work within the banking industry. The concept of a 'bank holiday' or 'public holiday' has no other legal significance in the workplace (unless the employee's contract states otherwise).
Your employee doesn't work Mondays, so you never have to pay her (or give her a day off in lieu) for public holidays which fall on Mondays. (If you've got other other employees who normally work on Mondays, but get the public holidays off, those days will come out of their annual holiday entitlement. Your employee who doesn't work Mondays will, of course, lose nothing from her holiday allowance).