Gas heating is by far the cheapest and if you are buying a house on the gas grid that is what you should go for. Oil is the usual alternative if you are off-grid. But it is more expensive than gas. Electric heating is also expensive unless you are buying a brand new highly insulated property, but it is still more expensive than gas.
The only difference between a combi boiler and and an ordinary one is the way it heats the domestic water. With an ordinary boiler, the hot water is stored in a big tank which is heated up by a timer, usually incorporated with the central heating timer.
With a combi boiler, the hot water is heated when you want it, a bit like an old fashioned geyser, except that it is combined with the boiler - hence the name - combi-boiler. The pros and cons centre on how you want your water heated as the space heating works the same in both.
With the standard boiler there is a hotwater tank, which is usually in the airing cupboard and you can dry and air clothes there. However, as you draw the water from it the tank refills with cold water and the water gradually gets colder and will not reheat until the timer switches it on again. With the combi boiler you always have hot water on tap, but you do not have that big warm tank in the airing cupboard because the water is only heated as it is drawn and is not stored and the water tends not to run as fast as with a hot tank, so filling a bath takes longer but it shouldn't affect a shower.
A better word than 'apologise'?
