Unfortunately this is near typical for most aquariums (and all Sea Life Centre's).
Personally I find them pretty poor value for money when you consider how long (even a very interested person) can spend looking around at these places. I've not been there but guess you'd struggle to spend more than two, three at max, hours there -which looks very bad when you consider that a small/medium zoo is typically around a tenner (for half a day's worth of "entertainment" and the larger zoos are roughly the same price (or slightly less) for a full day out.
I know there's sometimes vouchers out there that diminish the price a little (especially for family groups) but that's not the point -the "headline" entrance fee is appalling.
On a personal level I rarely visit these places. About every three years my memory fades and I go round one only to come out thinking, "why did I do that, I should have known better" and "I could have had a whole day at (insert major zoo) for the price of those two hours".
I suppose there is some argument that £17.50 for a couple of hours entertainment isn't bad compared with, say, the cost of 90 minutes for a Premiership football match, but conversely a couple of hours at a cinema is around half that -I'm amazed these places survive (and seemingly thrive).