Part of the problem is that EU companies are annually issued free carbon tax credits to a certain amount to satisfy obligations under the Emissions Trading Scheme - these can be traded or "cashed in" against ETS allowances for emissions.
The EU recently suspended free credit issuances to British companies, in preparation for Brexit, but did not release those companies from their obligations for meeting allowances, and thus British Steel now faces a large emissions bill rather than it being covered in whole or in part by free credits (previous years saw British Steel covered 100% by free credits, with an excess in credits that it could trade - those previous years excess credits would have covered this years bill, but were instead traded on the expectation that this years free credits would be issued).
This isn't wholly the EU's 'fault', as British Steel could easily have not traded excess past credits, but equally there was very little notice of the EU suspending free credit issuance to British companies, so this was potentially difficult to foresee. Equally, it was the British governments choice to remain part of the ETS beyond the original Brexit date.
