I was puzzled to note when extracting the above information from Google Flights that it doesn't recognise SEN as a London airport.
And that's why I never use Google Flights - it will only allow you to select 5 airports for the city - if you use the drop down and select more airports you will find that LONDON Southend is on there.
09.10 SEN €30 (yes, thirty euros. This is not a typo.)
And when you try to book it amazingly the fare jumps up 55.99 Euros.