For the first few years of service only US weapons will be integrated onto the F-35B as a cost saving measure in order to achieve IOC without any delays. In the RN anti shipping strike is primarily the province of SSNs first and ship based SSMs second, with helicopter launched AShMs third (mostly for smaller vessels e.g. FACs and patrol boats). Don't expect anything to happen regards AShMs on the Lightning until the mid 2020s at the earliest, as the aircraft are primarily envisaged as providing strike against land targets and against Airborne threats. The same is true of the RAF's new P-8 Poseidon MPA's they will initially only use the same weapons as USN aircraft with UK weapons integrated several years down the line.
I did say some in the RAF and their 'supporters' were dropping hints about a split buy, they've been doing that for years and haven't gotten anywhere. As I said although the A model is a bit cheaper, overall the economic argument is against such a move.
