tabbycat,
Specific aircraft are rotated regularly for operational reasons and maintenance so it's imposable to say this far out which one it will be. There are 16 737-800's (12 are the newer 73H's) in the fleet currently with a new arrival scheduled for delivery this month (G-FDZX), and the next 2 in October. They're all relatively new with the same interiors (apart from the 4x 738's) so it makes little difference if it's 2 months old or 2 years.
Darren