They used both (and barrage balloons as well). The space between the ~middle of the English channel and London was divided into "belts", with one type of defens

They used both (and barrage balloons as well). The space between the ~middle of the English channel and London was divided into "belts", with one type of defense operating in each belt to prevent interference (like flak shooting down your own fighters). At first the flak guns were located near London (as that was the same flak guns that were protecting London against aircraft). It didn't work so well because it was very hard to hit a fast moving target like the V1, as crews not used to it tended to use too little lead. And even if the V1 was hit, chances were that the wreck would fall down on London anyway. So eventually the flak batteries were moved to the coast, and they got gun-laying radars and proximity fuses which dramatically improved the effectiveness of them.