Replacement for Pearl Barley in 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.

The reason to include the barley is to bulk up the filling and use less meat:

all the flavours of a proper beef stew, while the barley reduces the amount of beef needed while still retaining all the flavour

If you want to stay close to the recipe (and it's up to you whether you want to or not), you probably want to use something that retains a bit of a bite and doesn't disintegrate into your sauce. Instead of barley, most whole grains should work, from wheat berries to rice, and I would pick wholegrain rice over white, given the choice. Considering that you need only 100g, you could - if you are really determined - ask your local bakery for a cup of whole grains, they might have wheat or spelt berries for some of their breads, or check an organic store if your local supermarket is too limited.

Lentils are always mushier, because they have a soft center in a thicker skin. Once that breaks, the lentil is prone to disintegrate. You can mitigate that a bit if you cook them with salt (contrary to the often-repeated advice to avoid salt). Flavor-wise, lentils are a common choice as meat substitute, so while you wouldn't be that close to the original, the results should still be good. Choose a type of lentils that is used for salads - so black and green (verte de puy) sound good. They will darken your sauce a bit, but that's fine considering the given ingredients. Do not precook them, if you are going to simmer them quite a while in the sauce as per the recipe - or if you do, only very briefly, five to ten minutes perhaps. You may want to adjust the lentils-to-liquid ratio though. The rule of thumb is one part lentils to two parts of water, while barley needs between 2.5 and 3.5 parts, so 150g lentils plus 550g water should give you roughly the same consistency as 100g barley + 350ml (absorbed) +250ml (leftover for the sauce) water. But that's just a ballpark number, cooking time and evaporation will influence this, so use common (cooking) sense and aim for the usual cottage pie filling consistency.

It seems that the pie is made with properly pre-cooked barley. 15 minutes should be enough for it to get well hydrated, even if the texture can be still a bit al dente at this point. This actually makes it easier on you, since it means you don't have to do complicated adjustments.

All you need to do is to choose your substitution, choosing an appropriate cooking method that doesn't produce excess water, or in the worst case, drain the water after cooked, and stop your method while the substitute is still slightly harder than you'd eat it. Then add it to the pie just like you would do with the barley, without any adjustments to other ingredients or liquids.

The last paragraph is deliberately vague, because the choice of liquid amount and cooking method will depend on what you choose to use as your substitute. As said in comments, lentils are not a very close substitute, and you can certainly do better. Most grains, and some pseudograins like buckwheat, will get you closer to the original (the smaller pseudograins like amaranth will simply lack the bite of the larger ones). If you do use lentils, green will be the best choice, especially if unpeeled.

I would suggest using small pasta such as fregula. This will similarly soak up the flavour, have a roughly similar texture, and cook in the stew. You can probably use the same weight.

If fregula is unavailable, consider smashing up some other dried pasta (e.g. spirals).

You could consider oats, similar texture or just omit either. It just seems like filler anyway. You already have the potatoes for carbs. This just seems like a starched filled redundancy.

Farro is a grain I like to use a lot that is somewhat similar in consistently to barley. And the cooking time / process is similar. So it would be a good replacement.