UK does not need a weapons bay because Stingray is electric (as will any replacement be) and its fuel does not freeze.
Putting aside all the arguments about carriage of external stores and their effect on range/performance, what would be the point of procuring a MPA/MMA without the growth potential which would be provided by a bomb bay? Who knows what sonobuoy and weapon configurations might be developed within the life of a new airframe. For example the lack of a bomb bay would severely restrict the optimum delivery of SAR equipment such as flares and liferafts unless the chosen airframe allowed them to be shoved out the door ala C130
Regarding carriage of torpedoes, while Stingray uses seawater as an electrolyte, it is by no means certain that the next generation of batteries will use the same technology. Who knows we might even have to resort to stored chemical energy power sources as used in the Mk50 or lithium boilers which were being developed in UK.
Even if Stingray does not have an electrolyte which might freeze, the parachute pack wont take kindly to prolonged cold soak!