The way Medway works is they test 3 subjects but give them diff weighing.
Maths, English and VR. The maximum score is also 140 in each paper too.
However Medway gives English and Maths double weighing meaning max score for these is 280 + 280 while for VR its just 140. The max score is 700 and to pass you need around 510 minimum
The major difference is Medway put all three scores together so if a child does very well in English and blows the VR or Maths they could still pass based on aggregate score. Kent is different in that you need to get a minimum score in each subject to pass.
You really don't need much prep if any at all in addition to Kent so your DC may as well just do it.
I think there are buses from West Malling to Medway. Depending on the schools, some Medway grammars are VERY easily accessible from Maidstone esp Chatham boys which is along the 101 route (every 15minutes). Look at the school here http://www.11-19travel.info/default.aspx
and see if the journeys will be doable. On top its covered by the Freedom Pass so you dont need to fork out more.