The answer to this question is complicated and multi-faceted but we believe that the right approach for our health and the environment is to eat a predominantly plant-based diet and good quality organic, grass-fed meat that has not damaged the environment (for those that want it).
We think The Soil Association sums it up neatly with their phrase ‘it's the how, not the cow’ (or sheep) that causes most of the problems.  The environmental impact of meat that is produced using organic and sustainable methods is very different to that produced by feeding livestock on damaging commodity crops or in other unsustainable ways. You can read more about the benefits of organic farming here.
A purely plant-based diet can in fact impact negatively on the environment.  For example, a soya burger needs soya - which does not naturally grow in the UK - and sunflower oil, both crops which will have been grown using fossil fuel-driven machinery. Both these crops also involve regular cultivation of the soil, causing damage to its structure and causing it to release its carbon.
You can read more about this complex issue in our blog here and in the following questions and answers.