I was in Morrisons Dundee and they had packs of two Red (only red) Polo Shirts priced marked at £1.50.
Various sizes from age 3-4 right up to age 13-14 which would possibly fit a small adult.
They maybe no good for school shirts, certainly for the Dundee kids as I don't know a Dundee School which uses Red Polo Shirts as part of their uniform.
They would be ideal for gym or even just play t-shirts at this price.
Not sure if it's national, but certainly worth a nosy if you are in a Morrisons.