Hi Martin
You can use beef or lamb, but neither works as well as pork when cold, (in my opinion). My Auntie Gwenn, proper Welsh she was, used to make a lamb and leek mini pie which was excellent hot, good warm, and a bit chewy cold.
Beef is a staple in this country and goes back further than pork in culinary history. Top of the tree is steak and kidney of course, but in the time of Charles Dickens for example, it was beef and oyster pie which fed the masses. Worth a note that the more opportunist pie men of London were prone to add horse, dog or cat or even kittens to the mix to vary the taste and texture. Cold beef is fine but needs something to bind into as it can be a bit dry.
Pork became popular because it was cheap and available to most. Outside the cities many had a pig to feed and fatten through the year. Once killed and without refrigeration it had to be dealt with within a very short time and every bit was used, hence the pies and brawn. Bacon was cured and hung and hocks were pickled and air dried with the bacon for use in the coming months.
Mouth watering, must go and cook.
Hope that helps.
David 