theres nothing that difficult about installing the radiators and pipework, its more boring than anything, although if its your first time you will probably find it really interesting!
the hardest bit is moving all the furniture, getting the carpets up and then the floorboards which is a pain, then putting them all back again of course.
one downside is there may well be some specialist tools that you will need, pipe benders, cutters, blow torch, core drills and powerful drill to match.etc which your going to have to buy and which need some skill to use, esp pipebending, its an art, i still haven't got the hang of it after 2 years training!
when installing your pipework underfloor you need to be aware of notching the joints at the correct depth, wrapping the pipe to prevent expansion noises and not then whacking nails thru the pipes when you put the boards back.
and make sure you dont mix up your flow and returns or put your thermostatic valves on the wrong way, which we see all the time, even from the professional installers. And although 8mm or 10 mm flexible copper pipe is easier to install 15mm is more professional.
i dont see any reason why you cant cut the flue hole or mount the boiler on the wall yourself but you MUST get the gas supply done by a corgi registered gas engineer and he must also fully commssion the boiler for you and register it.
one problem you may have is that you might find it difficult to get a gas engineer to come out and finish the installation for you, most of them like to supply and fit as they can make money on the parts, boiler and labour that way, they may also be reluctant to put there name to an installation they haven't done completely themselves as it has to be registered with corgi.