Just to go through the numbers, the EU are expecting to get 100m EU doses of 300m doses by end of June.
The UK has created vaccines for the 21m vaccinated with AZ so far; there have been 5m doses from India, so 16m. There need to be second doses before June so 32m plus say an extra 5m vaccinated with AZ in UK during April/May. That's 37m UK doses to be received by end June.
UK signed the contract for 100m doses and gave MHRA approval first so should expect to be a little ahead, yet does not appear to have received a significantly higher percentage of its contracted doses than the EU at a projected 37% UK versus 33% EU (based on a lot of assumptions about extra doses coming through that may not).
If the EU wants extra doses then all that could be provided would be to take the 16m second doses from UK + whatever 3m or so that could be left of the first doses, under 9.5% of their deficit and 6.3% of their total order.
Getting that approval presumes best efforts to mean taking doses in preference to a contract that was signed before theirs, I don't know Belgian law but that wouldn't work in UK law.