If you are serious about this you will need to get yourself a copy of the BS5839-6 which covers domestic properties and the regulations surrounding the requirements for installing and fitting such a system, if however you are looking to expand and go into bigger systems in multiple occupancies, industrial, commercial etc then I would also advise you get BS5839-1...
This would at least start you off on the right path, but as it sounds like you are not an electrician either, you may have an uphill struggle to understand many of the regulation as they do go hand in hand with other electrical standards and codes of practice.
In response to you opening post though, there is nothing stopping you doing what you wish to as long as you do not interfere or restrict the operation of the existing system as installed by your local council but I would strongly deter you from even attempting such as you may actually create a bigger risk to life depending on how one installs it.. a little knowledge in this field is very dangerous and ignorance of some or all the regulations can not be used as an excuse.
I will second that of other opinions here and express that this is a bit of a waste of money on your part and fitting a simple plug and play system which will not give you any real experience of fire alarm systems in the real world, their are many additional regulations now to consider too with the introduction of the BS7671 18th edition that dictates how cables are run and routed now in all rooms that may effect you install, your ignorance of this may put not only your family at risk but also that of the emergency services possibly entering the building while its on fire.
There are no real short cuts in this industry, I would expect anyone under my wing to have gone through electrical training + several yrs of onsite experience and have a full working knowledge of the minefield of regulations and practices associated with fire alarm design and install if they wanted to go and design and fit one themselves, I talk of course in exclusion of your basic domestic set ups which I could easily explain in a day for most cases.