How practical is it is use a 6 Terminals junction box to extend a ring main?
I don't know about the inspection issue, but personally I'd be wary about using a single 6 terminal block to extend the ring.

I assume you are breaking into the ring at a single location and inserting a loop to the new socket(s) you are installing. In that case I'd use two 3 terminal boxes at each end. Depending on the length of cable and number of sockets I might be tempted to install spurs, but that is just me.

However, I would suggest that you consult an electrician to see what the approved method is. The last thing you want is to have to redo the wiring when you come to sell the house. The easiest way is to get a quote and have the electrician explain what he's proposing to do.