I would not say that £1000 ($1600???) at all times is normal!!
I use an FTO in the US which requires you to put blocks of $750 on at a time, but you run these down to $0 before you put in your next $750. If your account reaches $0 then you dont fly until you top it up again, but whilst you are in credit you fly - Simple!
They give you a gentle nudge if you are getting close to owing them money so there is no danger there! :o)