There is a massive part of this discussion that is being missed here though a few people have picked up on it- livestock farming is not necessarily by default unsustainable or cruel. You find just as many plant/crop/veg farmers who are unsustainable and who are killing the environment via bad practice and pollution (particularly as crops and vegetables require high levels of fertiliser to grow them, and artificial non animal fertiliser is the MOST ENERGY INTENSIVE manufacturing process in the world (the haber process). Organic veg or crop farming is no better (infact I would say it’s worse as it releases much more carbon into the system from soil systems due to the reliance on the plough for weed control and uses highly toxic compounds such as copper sulphate for weed control in some cases!)
Our British farmers really are leading the world in sustainability- the new ag bill which is replacing the CAP is to be based on a system of enhancing natural capital, eg rewarding for high welfare livestock production, land management techniques that regenerate farmland and sequester carbon (such as trees and hedgerow planting, peatland restoration, conservation tillage).
It is almost impossible to grow crops and vegetables sustainably without livestock. Basic soil science taught at secondary school taught us this as one of the most effective ways to reduce soil degradation is to introduce grazing by livestock and livestock manure!
What we really need to encourage as consumers is sustainable agricultural practices, and when I say encourage, I mean provide a market outlet for produce produced in this way. By going vegan, you are removing yourself from being able to create that market demand for sustainable, ethical products- the easiest way to do this in Britain is to eat produce (both veg and meat) that carries a farm assurance sticker- red tractor or scotch which gives a guarantee of origin (born, raised and slaughtered in GB), and a guarantee of high standards underpinned by the mountain of legislation that farmers have to follow.
Our food choices can all help do their bit to make life a little more sustainable and ethical, but going vegan does not automatically make that happen, intact almost impossible to do so without livestock featuring within the production cycle.