Hi there. This deal is now back in stock. It was available before xmas (http://www.hotukdeals.com/deals/sony-xperia-xa-lime-gold-black-perfect-like-new-for-79-99-o2-2582025) and for a short time due to a glitch the price dropped in error to £49.99. Both colours are currently available and people previously posted the Graphite Black version came unlocked. It seems the Lime Gold maybe locked to O2.
If the phone arrives locked you have 14 days to return it to O2.
Can't see any codes available to knock the price down any further. Also no other offers seem to be available with the handset currently with O2.
As it is a like new phone it doesn't come in the original box. Instead all you will receive is the handset and data cable. Judging by what you should get in the box if you purchased it new you seem to be missing the headphones and plug. (https://www.youtube.com/watch?v=QUhW3TKPeVU)
Now back in stock as at 14:21 as at 10/1/17