Custom and practice
A. Establishing the right to something through custom and practice where there is no entitlement in writing is possible, but difficult. While the length of time is one factor that can be taken into account, it is not by itself enough to establish a contractual right. For the term to be legally enforceable it must be “reasonable, notorious [meaning widely known] and certain”, and your employer must have intended it to be a contractual right.
Guidance on this issue was given by the Court of Appeal in Park Cakes Ltd v Shumba and Ors [2013] EWCA Civ 974. The Court makes clear that it is for the employee to establish that a practice has become contractual, and says that they will not succeed if, viewed objectively, it can be explained equally as a matter of discretion rather than legal obligation.
It follows that the fact that your previous manager allowed this practice to continue every year does not necessarily mean that you are legally entitled to the time off, since they may simply have been using their discretion every year.