That looks like a bit of a long way round or don't you/the family fancy a nice long ferry trip from the Hoek to Harwich (or even Ijmuiden to Newcastle) ? Good way of avoiding the Thames crossing too.
BTW you will be near to one of the longest private railways in Britain - but it's freight only and mostly laid with cwr.
It is the long way round. But much cheaper than the alternatives, even counting the extra petrol and an overnight stay. We won't make the journey in one go, we'll be staying overnight in YHA Medway .
We'll I'll try to squeeze in a visit to Historic dockyard in Chatham in the afternoon, but negotiations are ongoing.