Not sure about Air Miles, but BA Miles work on the basis that you can book a free flight anywhere. The world is divided into zones, and you pay a set number of miles for any flight anywhere in those zones.

Zone 1 for BA is UK domestic plus Ireland and maybe a few bits of Europe like AMS or BRU. ANY return flight in that region is available for 13,000 miles. It can be LHR-EDI or MAN-GLA or whatever you want.

Zone 6 extends out to Australia. Obviously that will cost you a helluva lot more miles.

13,000 miles is what an Economy ticket costs you. You have to double that to get the Biz price and triple it for the First price. You could quadruple it for Concorde.