darnicmax - *if* this manoeuvre was made in response to a TCAS (mentioned earlier) system RA (Resolution Advisory) it most certainly is reportable under the UK's Mandatory Occurrence Reporting scheme. However, as others have said, the manoeuvre is safe and well practiced.
VSB via OL - I belive the matter of an airprox would be invetigated by the UK Airprox Board, rather than the AAIB.