Franbern, why don't you ask British Gas for a quote? They installed ours at what I thought was a reasonable cost. Unfortunately the previous boiler, which was relatively new, kept breaking down and I was fed up with it.
The work did involve changing piping as it wasn't compatible with the previous piping.
The cost varies, depending on what needs to be done. They work out how many radiators it will need to support, which determines the size of boiler. I have a Worcester Bosch which supports up to twenty two radiators.
The installation was supposed to take two days, but they did it in one day, because I had severe health issues at the time.
I was delighted with the service.