I would have thought there was no chance of the Competition Commission allowing that, unless they agree to divest either O2 or Vodafone UK, and either Virgin Media or the fixed-line business of Vodafone UK (I believe that includes City Fibre, which is very definitely a competitor to Virgin).
Maybe it will be like when Vodafone took over Orange, they didn't even attempt to get Orange UK past the Competition Commission, they immediately offered it for sale.