tayair 6
I cannot think why anyone told you that the A320 "can't be use(sic) at SEN" nor why specific CAA permission would be required.
Like any other narrowbody, provided it's perfomance is sufficient to carry the required payload to destination off SEN's runway and the RFF cover is sufficient etc., where would the problem lie?
I feel though that SEN certainly needs an additional operator or two to ease the current reliance on easyJet; perhaps news on this might be forthcoming before too long, although it's a very competitive market out there.