Heh...well I think your statement contains its own answers!
Common sense may well suggest, but common sense is in short supply when it comes to procurement of military equipment!
The RN wouldnt ask for something “inferior”, but it might be foisted on them for political and/or economic reasons.
This article seems to suggest that the RN was not too keen on the SM-2/AEGIS given that it was primarily focussed on countering sea skimming weapons.
http://findarticles.com/p/articles/mi_qa3738/is_199704/ai_n8769221
The SM-2 block-IV was cancelled a few years ago.