The collapse is rather difficult, but it is possible to fold a simplified version. The molecule uses a 10×7 square grid (with a few creases with half-unit coordinates). However, in order for it to actually look like the British flag, an extra unit of margin is needed, bringing the grid to 12×9. This margin can be shared by adjacent molecules.
This model was folded from a paper called Earth Treasure, which I reviewed in detail.