1. Those two quotes were not from British Gas. British Gas quoted much more...
2. I did not say forced. I said in many/most situations people are encouraged to get a combi boiler. I have found this myself as have many people I know. It is more often than not the first replacement option that is discussed. It is the most advertised type of replacement. The majority of boiler replacements are combis. Even when it is not the most suitable option you often find a combi installed (larger house with more than one bathroom).
3. Two other posters have stated that a simple retrofit of a modern boiler onto an old system is not that simple. I was also told this when I asked for a retrofit replacement option.
4. Regardless of the heat only option. It is a fact that modern boilers are less reliable. The vast majority (combis) have a life span of 10 years. As I said with savings of £300 a year and a replacement cost of £3000 it would take me ten years to pay it off. Even if I made £400 savings that is still 7.5 years payback.
My contradiction is the point I am trying to make. That spending money to save money does not always work out. It is not always the best option. If you have the money to spend and you just want to be as efficient as possible regardless the cost, that is fine, if I had the money I would do that too.
But to spend money when the primary aim is to save money (and it may take 7-10 years to make your money back and even then you may only just break even). I would ask myself if it was worth it.