Clearly it will be more difficult to increase commercial awareness of multiple industries if you specialise early. However, specialising in itself might play to your advantage. You can be the expert in your field of environmental sciences and be happy knowing you've gained some value from your education by consolidating your PhD. Despite this, I think gaining multiple industry experience may be what you are compelled to do in the first instance anyway. Without cunsulting experience, you'll have to get it from somewhere first (possibly a full service firm) before you're employed by a niche at a later stage. Does anyone disagree with this? In any case, good luck. As an aside, well done for attending one of the golden triangle of universities. There seems to be a misconception on this site that the first rank of universities is Oxbridge. Patently wrong - the big four in London are part of this group (UCL, Imperial, King's, LSE).