Why do they draw blood from a cow in front of the English Army?

Remember that scene in Indiana Jones and the Temple of Doom where they're forced to eat monkey brains?

Well, this is the Scottish version of that.

I've not seen the film, but I'd imagine that this is a "Welcome to Scotland, and this is what we do" sort of statement.

Black pudding is made from fresh blood. In modern times, it's pork blood, but in history, it was also made from cow or sheep blood.

Historical note:

Blood puddings are often supposed to be one of the oldest forms of sausage. Animals are generally bled at slaughter, and as blood does not keep unless prepared in some way, making a pudding with it is one of the easiest ways of ensuring it does not go to waste. While the majority of modern black pudding recipes involve pork blood, this has not always been the case; sheep or cow blood was also used, and one 15th century English recipe used that of a porpoise, in a pudding eaten exclusively by the nobility. Until at least the 19th century, cow or sheep blood was the usual basis for black puddings in Scotland; Jamieson's Scottish dictionary defined "black pudding" as "a pudding made of the blood of a cow or sheep".

The description here tells us that the blood is harvested at the time of slaughter, but it's possible to take amounts of blood from living cows. The Maasi tribe of Kenya traditionally nick the jugular artery of cows to decant a small measure of blood for consumption. The "Outlaw King" scene above probably alludes to this practice.