Hi Rinat89,
In the end I studied at LSE. I would have preferred to go to Imperial given the strong industry links of the course and the overwhelmingly positive feedback from alumni. UCL I can't tell you about, though I someone I know who studied at both UCL and LSE thought the level of economics to be better at LSE. It all depends what you want out of your course. If you want to have a high level of economic theory centred around all aspects of env econ (renewable & non-renewable resources, agr, env pollution, fisheries, biodiversity, forestry, common property) and be able to work for Eftec, NERA, Oxera, CECA, the Treasury, World Bank, DEFRA (as an economist - they are now recruiting for economists to shape UK's post-Brexit fishing and agriculture policies) then go to LSE. I was initially accepted for UCL EPEE and I in fact bought a number of the books on the (pre-)reading list. Judging from the books that I read, I think you will get a good 'idea' of economics and policy at UCL, but if you want solid theory go to LSE. Also the lecturers at LSE are 2nd to none, look up their papers in Google Scholar: Ben Groom (discounting), Giles Atkinson (sustainability), Susana Mourato (valuation), Simon Dietz (climate change adaptation, sustainability), Charlie Palmer (forestry) and we have had guest lectures from Christian Gollier, Kirk Hamilton, Jeff Sachs.... It's an economists' paradise. The options are also interesting - you can pretty much choose from across the board at LSE, though Econ courses require you to pass a special exam. I have chosen development & social policy courses, but others have gone for health policy, behavioural economics, spatial analysis, urban development, int relations.
It may seem that I am promoting LSE, but I am not an economist and I would still have preferred to go to Imperial, to get more of the practical industry experience and sometimes I sit through some classes wondering when I will ever use this stuff, but I wouldn't say that I regret it....
Hope this has helped somewhat...