Originally Posted by NickB

Their obligation is to reroute "at the earliest opportunity". Whether that means at the earliest opportunity on their services or at the earliest opportunity on another carrier is debatable and, AFAIK, there have been, to date, no judgments to clarify this.

As to which national enforcement body (NEB) is responsible, it is normally the the NEB of the country of departure when the departure is from the EU, so the Belgian NEB rather than the UK NEB for a flight departing from Brussels.

If you were to take it to court, you would have a choice of jurisdiction between country of departure (Belgium) or country of destination (UK) to start proceedings.