Nobody can be strictly 100% vegan. That is unless we grew our own food, didn’t accidentally step on insects, and accounted for every way we could cause animal suffering. But does this mean we shouldn’t be 99% vegan? In other words, does this mean we should not try our best to avoid harm to animals? Absolutely not.
Believing that because we can’t be completely vegan, we shouldn’t bother at all, is a version of the “al tu quoque” fallacy. If a vegan says “it’s wrong to kill cows”, the response is usually “well, you step on ants, and drive through roads which contain animal by-products, so you’re being hypocritical”. This attempts to invalidate veganism by suggesting that there is no difference between a vegan contributing 1% to animal abuse and a non-vegan contributing 100%. This is clearly not the case; there is a huge difference.
In reality, the meat, dairy, egg, wool, fur, and leather industries alone probably account for 99% of all animal abuse. In our modern world, it is impossible to exist without coming into contact with some sort of animal-derived ingredient. So the fastest and most practical way to end animal abuse is by boycotting the big producers of animal abuse.
The main reason we find animal by-products in so many things is because of the scale of animal industries. They produce so much waste (ligaments, bones, brains, intestines, etc), that it makes economic sense to use it elsewhere. Decreasing the production of animal products, by avoiding the main industries, would make the use of these waste products impractical.
Nobody would produce tallow to put in £5 notes if it wasn’t already a by-product of making meat. So try your best, and live 99% vegan. Soon enough it’ll be 100%.