She was only built with a 20 year hull life, so any significant extension would almost certainly require an expensive amount of maintenance and refit.
This article from 2016 gives some background, and makes a case for retention of Ocean, pending a replacement. With 2 new carriers and manning levels fixed, it doesn't seem likely, even before current finances (or later ones if you factor in predicted economic impact of Brexit)
http://www.savetheroyalnavy.org/the-campaign-to-retain-and-eventually-replace-hms-ocean-starts-here/