If christmas falls at a weekend, as it does this year, and you would normally have the day off, it will be specified in your employment contract whether you are entitled to the following Monday off.
Most employers do allow for this and generally it is common practice, however there is no statutory right to time off for bank or public holidays. There is also no legal right to transfer leave entitlement from one day to another if this is not specified in your employment contract. If you are concerned about this issue, consult your contract of employment or staff handbook (if you have one) - you may also wish to discuss this with your employer. However, you should beware that your employers decision is final in this situation. When approaching the subject remember that if it's not specified in your employment contract it's is a request, not a right.