I am making a cottage pie (mince with mashed potatoes on top) recipe that requires 100g of pearl barley, and 600ml of stock. Right now,  I can't get either pearl barley or husked barley. The first step is place the barley, stock and salt into a small pan and boil for 25 mins, and it gets about another 1hr of cooking after that. I was hoping lentils would be a good alternative, I have red, green and black, but what would that do to the amount of liquid and that first cooking step? I found the recipe online: https://foodism.co.uk/recipes/yeo-valley-beef-barley-cottage-pie/ 
Cottage pie is essentially a ground beef stew with mashed potato on top, and barley is a completely optional ingredient in beef stew, so you could leave it out and not worry about replacing it, there's already plenty of starch from the potato topping. I make beef stew relatively often and I like barley in it, but if I run out I just leave it out and it doesn't make that much difference. The big issue with a substitution is flavor - barley isn't flavorless but it isn't what I might call strong either, it tends to absorb flavor from the liquid around it. Lentils have a comparatively strong flavor, as do many other grains and pulses. Rice is the closest easy to find substitution and it won't effect your flavor profile. If you do decide to use lentils I'd suggest making a test dish to get the timings right and make sure you actually like the taste of the result. 