It depends. Presumably a twin engined helicopter would have no trouble flying on top of this type of weather (am assuming radiation fog which isn't too thick) whilst enroute?
The opportunity of "flying on top" when in central London is probably pretty limited, whilst the weather was bad enough that weather was a cause of a divert. In fact its a pity that posts keep getting deleted because someone posted here that actually initially the request was to return to Redhill but that seemed not possible and so Battersea was 2nd choice.
Did the weather degrade hugely in those 25 mins?