LHR FCC is brutal at enforcing the limits, there is zero chance of getting 2 bags through per pax, even if you are flying F. Presumably you are flying CX J, and not BA J, thus you cannot even use the FCC Fast-track -- it is for BA J/F only. Plan on spending an hour in line at LHR security, and if you have 2 bags, spending another hour going landside to check it as baggage.