<<Sounds maybe a little bit stupid but why did they divert to STN?
LHR is not that far away and I assume better prepared for such damm occurances>>
There has been provision for many years for crews of a/c with "problems" to be asked, on behalf of the airport authority, if they would consider diverting a less busy airfield to prevent disruption at a major airport. The final decision is, of course, with the Captain.