you can try, but don't expect agreement.
If you do a general modernisation, including cleaning and filter, TRVs, programmable stat and upgraded insulation, you will save more than if you just change the boiler. In my own house, I had already done that, and changed an old iron boiler for a modern condenser (conventional, still with a cylinder) and the average annual gas usage (measured in cubic metres per year, not by the ££ on the gas bill) dropped by over 20%, which I thought very good.
However when you compare the saving on gas and the cost of installation, there's not much in it, so it's not a paying proposition to change until the old one goes wrong and is irreparable.
If you buy a new boiler tomorrow, there is about zero chance that it will still be working in 15 or 20 years.
Some people think that all modern boilers are combis, but that's not true.