"What would be interesting would be if they are Duty Free's or 'Counterfeit Afghan Market' (even cheaper). I don't think there's a limit on the latter."
There is. Theatre orders state that the limit is 200 cigarettes per serviceman, per journey, no matter where or how purchased.
If you wanted to go by what (as a civilian) you're allowed, UK customs wise, then yes there's no limit (other than that it has to be for personal consumption) but you have to provide proof of purchase and proof that they are tax paid. Your chogie cleaner at BAS won't be able to provide you with either. So either way the bottom line is, it's an allowance of 200.