The Gas side of it isn't the only bit you legally need an RGI for, flue too.
Other than that, feel free to do the rest of it yourself, no reason why not if you have the confidence. It might take you 3 times longer than any of us but you'll save the cash, which seems to be your sole aim.
Where is your gas meter in relation to where you plan to instal the boiler? You should also be thinking of the route for that, not just the wet pipes.
saving cash is important, but making a neat job of it is more important. with oak skirtings/doors etc, i'd rather take my time, and be happy with the job at the end of it.
As for the flue - i'm correct in thinking that the flue can be retro fitted on most boilers ??? the location of the boiler will leave pleanty of room for the regs as to windows etc. the flue will only need to be a foot long. every house in my row has the boiler fitted in the same place, its really the only place suitible for a boiler.
as for the gas meter ( it was only installed a week ago, at the cost of £500) - its in a good place, its mounted on the same wall as the boiler will be, so its straight into the house, up the channel through the concrete floor ( it carries EVERYTHING, h+c water, mains power, waste, hdmi/audio cables...)
I'll employ the gas guy to fit the pipe from the meter, T'eed, one to the boiler, then a run past the cooker ( elec at the moment) and then a potential gas fire location. I presume i can't run this pipe myself, and just have him inspect and plumb it to the meter ???