We can do a direct price comparison with our European neighbours, see what they're paying .
It's not that hard.
As it is, UK energy prices have risen along with those in the EU .
But UK prices
have risen more sharply, but that may be for reasons outside of brexit at the moment
However in the future being outside the EU may cause problems, or extra price increases.
https://ukandeu.ac.uk/explainers/energy-prices-and-supply-in-the-uk/
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"" What is the Internal Energy Market?
Even if the present energy crisis is not a Brexit issue, the place of Great Britain outside the single energy market might still cause problems in the future (Northern Ireland remains connected to the Internal Energy Market as it is part of a separate Single Electricity Market with the Republic of Ireland).
The Internal Energy Market, created in 1996, was intended to disrupt the monopolies in energy and gas markets in many EU and EEA countries by increasing competition and thus reducing prices for consumers. Over time legislation widened its scope: it now allows for the sharing of energy supplies between EU and EEA member states."