You cannot pose a question such as this one about the entire UK because we are covered by three bishops’ conferences: England & Wales; Scotland; Northern Ireland (shared with Republic of Ireland). You need to ask the question separately for each one.
It is not a holy day of obligation in England & Wales. I cannot find any information on either the Scottish of Irish bishops’ websites. I cannot, therefore, tell you about them. I would not want to guess because I know the three have different holy days of obligation.