I wonder if it's another pillar 1 vs pillar 2 thing but can't see anything to suggest that.
Yes, it's in the "About the Data" tab:
The UK total is not the sum of the 4 National totals as the pillar 2 cases cannot currently be included in the individual National totals for England, Northern Ireland and Scotland. All other data on this website are based only on cases detected through pillar 1.