nearly all builders merchants will sell the round soffit vents you won't have to comply anyway but the requirements for a new build would be 25mm continuous vent allong the whole soffit or equivilant.
Works out at 1 in every gap between rafters roughly. but as you are only trying to ventilate your roof space and not a void behind insulation I would remove the bricks as you suggested probably in every other gap and fit a vent to those gaps at the soffit
Works out at 1 in every gap between rafters roughly. but as you are only trying to ventilate your roof space and not a void behind insulation I would remove the bricks as you suggested probably in every other gap and fit a vent to those gaps at the soffit