Yes but London Gateway is much closer to Greater London than Felixstowe so potentially the driver can do a lot more in their working hours. I'm not so sure that the difference is marginal.
But you need to ask youself this question. Where do the majority of the population live? The South East. In which case containers heading for the SE will land at london Gateway, in which case the majority will go by road! Intermodal runs from Daventry runs to Cardiff and Scotland, its not economical to run it to anywhere else. Why do you think that London Gateway will change any of this?
Not only that but its hard to believe that changing ports is likely to generate new freight traffic from distribution depots. Its hard to imagine Daventry closing because distribution depots have opened around London Gateway. Felixstowe to Daventry is probably quicker by road than London Gateway to Daventry.
In terms of timings Felixstowe will always be quicker unless the final destination is the SE. In which case the vast majority of containers will surely leave London Gateway by road?
The population density is greater in the South East but it is not true to say that the majority of the population lives there. It's about 8 million in London and a similar figure in the rest of the region, so less than half the population even of England.
The places that are within a day's out-and-back from London Gateway and not Felixstowe would be the Thames Valley and surroundings. There aren't any intermodal rail terminals there so they must be served by road already. In any case any distance advantage of London Gateway is to a large extent cancelled out by slower traffic around London.
On the other hand there are places in the Midlands that are within a day's out and back from Felixstowe but not from London Gateway, so any shift to London Gateway is potentially an opportunity for rail.
Combine this with the fact that each shipping line will want to serve the whole of the UK with as few port calls as possible (probably only one) and I think it's clear that the proportion of containers where rail might be competitive will be at least as high at London Gateway as at Felixstowe. Thus, if rail can cope with it, at least the Fexlistowe market share is on offer.
The advent of a new port and distrubution centre close to London might prompt existing or new distributors to set up Daventry-type operations there if the economics prove attractive. Given that the distance to Scotland is further than from Daventry, it's reasonable to expect that these might generate domestic rail flows to and from Scotland - again assuming that rail has capacity to handle them.