The British Army has the following information on their website with regards to pension:
Every month, the Army pays into a pension fund on your behalf. And when you retire, you will receive a monthly payment based on your final salary.
When you join the Armed Forces, you will automatically be enrolled into the scheme – and you won’t be asked to pay a penny
After two years of Regular service you’ll have earned an Army pension that will be paid when you get to the age of 65
Anybody aged over 40 who has served for at least 18 years gets the right to claim an immediate pension linked to their final salary, a tax-free lump sum on leaving the Army and a second lump sum when they turn 65
The pension scheme will change on 1 April 2015 and from this date Reserve Forces will also be automatically enrolled
It may be worth confirming this during interview or on having a job offer, just to be absolutely sure of what you personally will be entitled to as it may be partially dependent on the specific role that you are taking on.