The thick incoming cable that is fed from the meter thats outside - also goes off elsewhere?
So where? Are you also supplying a neighbour and is that neighbours usage through your meter?
I don't know yet, but I'm pretty positive there's no issues with sharing electricity and metering. It's not an oddball house or a weird setup in general, it's just a simple terrace that seemingly hasn't had anything done with the electrics this century (as someone said).