In general, with alternating current, AC, systems the current does go backwards and fowards all the time, but that does not mean that it goes one way around the ring main and then the other. In fact it should never go around the ring, because as you pointed out in another answer the potential is going positive and then negative on all parts of the ring at the same time.
So if nothing is connected to the sockets on the ring main almost, but not quite, zero current will flow into the ring and out of the ring the the voltage goes positive and then negative in the ac cycle. You are correct in your comment - the 'potential' or voltage of the wires will go up and down.
Now if you connect an electric heater and turn it on then current will flow into the heater and then out of the heater for each part of the ac cycle and current will flow through the ring main - you can think of it like a river with a large island in the middle and narrow channels each side of the island - water can flow around the island both ways and in the same direction on both sides of the island - how much flows on each side will depend on the length of the channels and other factors..... except becaue the current is ac in our model of the island in the river the water has to flow downstream - and then upstream... this could happen in tidal river where the direction of current can change due to the tide coming in or going out.
Now when we think about ac current it is aways going one way and then the other and if we took an average of the current we would always get zero, which is not very helpful. So to measure ac current we normally look at the `rms' value... the root mean square - we square the current so that both positive and negative currents give a positive value, take the average and then take the square root. So if an ac device needs 5 Amps of current then that probably refers to the rms value.
So when we talk about ac current it is always changing direction, but if we use rms values we can actually get a useful quatitative measure for the ammount of current going backwards and forwards in each cycle.
... sorry now rather long answer.. hope it is useful