Combi boilers are no more efficient than standard boilers. If you want to upgrade your boiler to one that's more efficient thats fine, but you don't have to have a combi. here's why.
1. You won't have to get rid of your existing hot water cylinder and feed tanks, which will speed installation & save money and make less mess.
2. Standard boilers are cheaper to buy than combis (because combis include gubbins inside that allow the removal of the cylinder and tanks you already have)
3. Assuming you have modern controls with motorised valves, room & cylinder stats you can keep them as well. Have a combi and most will have to be removed. De-commissioning wiring is not always easy, install a standard boiler and it would probably be a straight swap of one cable.
4. Combis are more complicated than standard boilers as they have more components subject to failure. They are not suitable for DIY work, repairs should be carried out by suitably qualified people and with parts from the manufacturer. On the other hand, standard boilers have a lot of components externally mounted. Pump, motorised valve, programmer etc., these can be easily replaced by a competent DIYer or local plumber, and with any manufacturers parts obtainable from local plumbers merchants.
5. Combis are slower to get hot water to the taps if the boiler is starting from cold when a tap is turned on. You could of course use the preheat setting that fires the boiler every so often to keep it hot just incase you need some hot water (a bit like starting up your car every half hour to keep it warm in case you want to go out)
6. With a standard boiler, you can have an immersion heater to provide hot water when the boiler fails (and they do fail, read this forum) With a combi its the kettle & saucepans.
7. You will have a lovely warm cupboard to warm towels, air clothes, make jam or wine etc.,
8. Just because everyone else is having one doesn't mean you have to or it's the best way (think endowment mortgages in the 1980's
)
So why the combi? The advantages are primarily related to the installer (not the user) in brand new homes or where no system is presently installed because a lot of the components that used to be external are contained inside. This makes it easier/cheaper to install from scratch, and for house builders, no airing cupboard means new houses can be built even smaller.