I'm not from the UK, and admit that I'm not famiiar with this program. Is this the program you are talking about
:
http://www.airmiles.co.uk/ ?
I took a look at this website, and I see no way for you to earn miles by flying. It appears that the basic categories that earn miles are:
Groceries, Credit Card, Home Energy, Petrol, Online Shopping.
Looks like miles earned from these purchases can be exchanged for flights, but flights don't earn miles the way they do in airline frequent flyer programs.
And in general, I would be very surprised if any rewards program will give you credit for any activity that took place months before you joined the program.