In the UK there are currently three main types of Gas heating and hot water systems. Checkout the Gas Appliance Guide site for description with pro's and cons. http://www.gasapplianceguide.co.uk/central_heating_systems.htm

When attempting to turn off your water in your home you need to generally know what type of system you have. You need to know if you have:

1 A mains system (either a high pressure or a combi boiler) or,

2 A gravity fed tank system.

Gravity fed. If you have a gravity fed tank system as well as the boiler you'll also have a hot water cylinder in a cupboard somewhere and also much higher up (like in a loft) a cold water feed/expansion tank(s).
With these systems you will usually only have mains pressure to your cold water taps, so turning off your mains will only shut these off. The hot water taps will be fed from the tank in the loft which flows by gravity down the hot water cylinder and forces hot water out to the hot water taps.
Some systems also feed cold water to bathroom cold taps and the toilet cistern so the only mains water is going to the cold tap in the kitchen.

If you are doing work on say the hot water tap in the bathroom you need to turn off the mains first, then you'll have to run the tap until the tank in the loft empties. You don't have to do this if you can find a stop cock coming from this tank. Simply turning this off will stop the flow of water. This is actually much more preferable because if you do have to drain the tank you may end up with an air lock in the pipes which can sometimes be a bit tricky to get rid of and means you'll get very little water coming out of the tap!

Some systems incorporate a pump to help provide greater pressure at the taps. In residential areas with low water pressure a gravity or gravity/pumped system is usually the best option.

Combi Boiler. A relatively new type of heating system. You will normally find them in smaller properties with limited space. The benefit is thy require no feed tank or hot water tank. Hot water is made as you turn on the tap. All your water including the hot is mains fed so you have good pressure throughout.
You only need to turn off the mains stopcock to turn all the water off.

Advantages are that they are compact, require no hot water or feed tank, and you get mains pressure to all taps.

The disadvantage of these types are that flow rate can be limited so not suitable for multiple heavy demand (eg homes with several bathrooms.) locations

High pressure system. These systems tend to be more expensive to install but provide mains pressure water to all taps and they tend to give good flow rates of hot water.They require a hot water tank but no cold water feed tanks in the loft.
The water pressure is related to your incoming mains pressure so if you live in a residential area with low pressure then they are NOT a good option. Being pressurised they may require more maintenance and some require safety certification when installed.

As with any system that stores hot water in a tank you are making hot water regardless of whether you use it (unlike the combi option which only makes hot water on demand)