A Christmas Dinner or Lunch is an allowable company expense as long as total expenditure does not exceed £150 per person and it is offered to all employees.
Which Tag should be used for this, ‘U.K. Entertainment’ or should there be another tag for “Annual Event” in order to show compliance to HMRC qualification requirements?
I would use a “Staff entertainment” code (rather than Business Entertainment which is disallowable for tax purposes). If you find it easier to keep the annual events separate, in case you have more which put you over the £150 per head annual limit, then it’s up to you. If you only have one event and you’re always within the limit, then there’s no need to separate it out.