If it is time to change your boiler, then the first thing is to decide what type of boiler you need.
If you have mains gas, then a gas boiler is usually the cheapest heating system for you.
If you currently have an oil, LPG or coal boiler, then it might be worth looking into the cost of getting gas supplied to your home. If there is no gas pipe near your home then this is not an option, but if it's available just round the corner then it might be worth paying for a new connection to get cheaper fuel.
Most old gas and oil boilers are regular boilers - they have a separate hot water cylinder to store hot water, rather than providing it directly from the boiler. When you replace your boiler you have a choice of buying a new regular boiler, and keeping your hot water cylinder, or buying a combi boiler that doesn't need a cylinder.
A regular boiler is actually more efficient than a combi at producing hot water in the first place, but then some heat is lost from the hot water cylinder, so a combi may be more efficient overall. The best option for you will depend on a number of factors: