Is it a legal requirement / building regulation to have a stopcock within a property?
If it is a requirement to have independent control of the water supply now then it won't be applied retrospectively unless and until you have some work done on the water supply.

The first thing you should do is arrange with your neighbour a mutually convenient time to fit individual stop cocks. Then when this is done you can then arrange with the water company to fit the water meter.

Having said that, we have a house built in the 1890's that has it's own stop cock so it might be that you just haven't found it yet. Have you checked close to the house?

Another possibility is that the stop cock has been buried in a floor. This was the case in our previous house where the kitchen floor had been relaid and the stop cock - which was just below floor level inside the house - had been partially buried.