BA's new 321's have 'flexible seating' so they can change the club class/euro traveller configuration depending on the booked loads. In an all 'euro-traveller' (ie economy) configuration the aircraft holds 194.
The 757's also have flexible seating although the maximum pax it can hold in an all economy config is 182.
's 757's were delivered with a galley in the front, middle and rear of the aircraft (nice for the crew but at the cost of bums on seats). The A321's however only have a front and rear galley so freeing up more space in the cabin for seats. It will have 4 toilets and wardobes at the front and rear.