I believe that quay extensions on the Felixstowe side are now at an end. Current work is concentrating on enlarged stacking yards.
Growth will therefore depend upon container handling efficiencies which (in view of up-thread comments) may be considerable. Also there is scope for transfer of traffic from road to rail, rail being less than 30% of current throughputs.
Hutchison Ports has had planning consent for Bathside Bay for some time. The last that I heard the consent, after the most recent time extension, requires a start on site by 2021.
The planning process has suffered appeals on the basis that the original permission was granted with the backdrop of the assertion that extra port capacity was urgently needed in the national interest. Delay in implementation would tend to defeat that assertion.
Anecdotally it has been suggested that the cost of ongoing dredging to afford the latest generation of super container ships access to the new quay may make the project uneconomic.
But of course that could just be fake news to mitigate and disrupt the actions of project detractors!
The Bathside Bay planning consent requires Hutchinson to make a very large contribution toward dualling the A120 from Harwich to Horsley Cross. Hence they're trying to extract the maximum possible usage from Felixstowe before starting work this side of the river.
They were also delaying major investment here until they'd seen the impact of the London Gateway port and the improvements at Tilbury. These don't seem to have had as much effect as they thought though.