from a purely legal perspective, i believe that you can do everything but the gas - you can't even run pipework for that. any involvement whatsoever with gas MUST be carried out by a CORGI registered engineer. they must do the work themselves and are not allowed to certify work of others who are not properly qualified. to do so is risking their own certification and possibly a jail sentence - which, by the way, you also risk if you work on gas without the necessary certifications.
there are also very many rules regarding siting of flues, pressure reliefs, drain-off for condensate etc.
i'd advise that you read between the lines of htg's response - if unqualified then it's unwise to get involved - call an engineer.
All sounds a bit "doom and gloom" doesn't it. I built this house I live in which is out in the country. I have a lifetimes experience in the building industry, and I am competent in every trade except plastering. Pretty poor at that.
I designed and installed my current heating system which is oil fired. Oil has become so expensive I began to think about converting to gas which is now available.
Being a crotchety old sod I don't like other tradesmen's work. It is never good enough for me. So I will now consider my options.
I have a letter in my and here from British Gas apologizing for their 35% increase in gas prices this July. In the past when I have done cost comparisons between the two fuels, there has been very little difference to choose between them and I am not sure that has changed any. Ok, the price of oil has risen sharply recently, but so has gas. The price of oil is now on the way down, but that is not reflected in my gas bill!
Frankly, there is no way I would spend money changing from one fuel to another. Maybe I am wrong and gas is more economical, but who knows? If you want to spend money, use it to save energy on such things as better insulation and heating controls. What about a more efficient boiler? A gas condensing combi perhaps?