I guess it depends on if it is a gas or electric shower.
For electric showers: Assuming an 8.5 kW shower running for a 10 minute shower and your electric cost is 11p/kWh
That's 1.42 kWh used which equates to 15.62p.
This does not take into consideration the cost of the water if you are on a meter which is £1.294 per 1000 litres.
If the output it 1 litre every 10 seconds, thats 60 litres so an additional 7.8p for the water.
So, a 10 minute shower in an electric shower costs about 23.42p
Oh bloody hell, just noticed you specified gas. Back to the drawing board...