For all it’s constraints Southend is in a very different place to Manston. People might critique Southend’s location and accessibility but it is within an hour’s journey by road or rail from a sizeable area of London’s administrative boundary. 45 mins to Stratford on a relatively inexpensive and regular train is a somewhat better proposition than 1h20 on a less frequent and quite expensive HS1 service. Southend’s immediate local catchment is wealthier than Manston’s. Therefore because Southend has attracted EZY, FR and others does not mean Manston can replicate that feat
The train journey is 1h20 because it isn't high speed after Ashford. It would need upgrading to HS in order to cut travel time. Saying that if this was a freight airport and that freight travelled on the railway at night it wouldn't really matter whether it was HS or not. I do think Manston needs to concentrate on freight to make it work. There is no need to limit it to night flights if there is no daytime traffic to worry about but that would go for the other end too. Fedex fly from the USA during the day currently so if they can find enough business after Brexit to make it work then who knows.