You get terminal grant at 36K plus 27K and then 9K a year until aged 55.
Then it reverts to 12k per year plus the total of annual RPI increases over 15 years (assuming leaving at 40) normally a 50-70% increase
as an aside.. that figure of £27k to commute looks high
my terminal grant is £42k plus £16k commute leaving £12k a year to 55, are you perchance an officer leaving at 37 ?