Plenty of boxes on the promotion aisles in Staines Sainsburys. It is not on their website so not sure whether it is nationwide.
It is generally available in pack of 12 or 24 and occasional 36 promotional pack!
From Sainsburys site:
THE WHOLEGRAIN CEREAL.
Low in fat.
Low in sugar.
High in fibre.
Whole Wheat Cereal With Added Vitamins & Iron.
Weetabix, the nation's favourite cereal, makes it easy to have a healthy and nutritious breakfast for the whole family.
Nestled deep in the heart of the English countryside, Weetabix have been creating delicious breakfast cereals since 1932. Using only the finest ingredients, all our wheat has been sourced locally, mainly from farmers within a 50 mile radius of our mills.
This pack contains 36 individual servings.
CONTAINS FOLIC ACID.