Passage accross from Belgium pretty strait forward. Cross the TSZ, head for Margate (keeping north of the Goodwins) then take either the Princess Channel, Kentish Flats windfarm to Port, Red Sand WW2 forts to Port, Medway approaches, Sheerness, river Medway. The nearest marina for petrol and diesel is Gillingham marina, Ch80. Speed limit in the last mile only! Chatham marina also do diesel and petrol but only upto 7PM. Beware their petrol price is HIGH (£1.21/L 2 weeks ago!!)
If the tide is with you, there is a Tesco just behind the sea wall at sheerness!
If weather is poor, take the inshore channel via Margate South channel, Gore channel, Whitstable street PHB to port, The Swale (inshore of IO Sheppey), Queenborough then carry on up the river Medway to Gillingham. Gillingham marina is about 15 miles upriver from Sheerness. Leaving the Medway, take the River Thames up to London. Berths available at St Katherines or Limehouse Basin plus a few others. No speed limit but watch your wash. Ch 14 London VTS to transit the Thames barrier.
Hope that gives you a taster. I would reccomend going via the forts at approx 51 28 N, 001 00 E.
Have done most of the route at some time, but not in a single run. If you need more info then PM me.
Searider - The Best 5.4 x Far