WHSG was my first choice as I live almost next door, but I think that's unlikely since we are no where near previous years cut off, so hoping for SHSG
I can't remember a year when WHSG has been oversubscribed in-catchment - unless there was a huge increase in the number of IC girls passing this year, your DD would just need 303, the same as for SHSG.
You can still put Westcliff and / or Southend on your CAF (just tick 'yes' where it says,'I believe my child can pass the entrance test', if that Is still on it). I'm March, your application will be rejected, but at that point you are able to appeal for a place. You can also put your DD's name on the continuing interest list (for SHSG this used to be done by making an 'In year' application via Southend Borough Council; not sure whether you still do it this way, or what the system is for Westcliff, I'm afraid). Then, if girls leave the relevant year group after a certain point in the year, your DD and the others on the list will be invited to sit an entrance test. Admittedly, there's no guarantee with either the appeals process or the in year application route, but my DD is one of a handful in her year at Southend who gained a place through an in-year exam.