The Government’s Energy Technology List (ETL) is a highly recommended resource. The ETL was devised to complement the Enhanced Capital Allowances scheme (a meagre tax concession for installing energy-efficient equipment) but it is useful in its own right because products cannot get onto the list without rigorous scrutiny from expert assessors. This makes it useful for filtering out bogus products.
Of course not every manufacturer, supplier or product that could be on the list is necessarily there. But the ETL itself, which lists products, is complemented by an Energy Technology Criteria List which lays down the criteria for inclusion within each product category. This will help you establish what to look for if you are doing your own evaluation of unlisted products (or if you just want to know what attributes are important, or what performance thresholds separate good products from the rest).
The ETL website has recently been revamped and can be accessed at https://etl.beis.gov.uk/.