Call me a cynic but maybe it's no coincidence that HAAWC, Harpoon II+ER and SLAM-ER are the weapons range on offer with the Poseidon and they're all Boeing products.
Certainly I would have thought that using pre-existing Sting Ray torpedoes with HAAWC would be cheaper than buying brand new Mk 54's but maybe not as Boeing could control (fix) the cost of integration.
Harpoon II+ER kits will be available in 2019 and will surely equip our Poseidons in 2020
https://www.janes.com/article/77914/har ... 019-budget. Once they're in service with the RAF then I doubt the Navy would be allowed to buy something different and create two small orders with separate maintenance contracts etc.
And here's what the Minister said
https://www.parliament.uk/business/publ ... 16/144839/
As for the future I can't believe once we've got the Mk 54 we'd then go to the expense of replacing them.
But I think there is a definite case for integrating Storm Shadow instead of SLAM-ER.
For the longer term there's the Anglo-French FCASW from 2030 so we'd be mad to go for the LRASM which is a direct competitor.