Assuming the work was carried out over 4 years ago your local council cannot take action against you for non-compliance. This does not mean to say that you should not have got BR approval - the builder who installed the RSJs was lying to you.
You could apply to the Council for "Regularisation" of the works but it's probably easier to pay for your buyer to employ a surveyor to approve the works.
Any bulding works tht require consent, in this case structural alterations, carried out post November 1985 will require regularisation by the local authority. The regularisation is to ensure the works complied with the building regulations at the time the works were undertaken, but in real terms for a structural alterations.
There is no 4 year limit for building control, sorry.
You may also find that if calculations were not produced at the time the works were undertaken, the building inspector may require these to prove that the beams are acceptable.
A call to your building inspector at the local authority will confirm the above.