No point comparing different parks as to how many rides they have open because location, park layout and potential popularity are all key indicators...
Southend is essentially smaller, less 'complicated' rides (compared to the likes of Vampire, what's the most complex ride there? Rage) and also is in the middle of Southend, meaning it's in a decent interest of getting people in as there's not exactly much else to do there in Winter...
The thing is it suffers from a similar situation to Towers, in that it's not particularly obvious to those who don't research (I.e. most guests) further into events... Therefore people go and assume the entire park is open (because the thought process is probably "oh it's cheap let's go")...
Although with a name like Zoo Days, would you expect the rides to be open? And I doubt they expected to hit capacity at all this week...