This will all depend on the situation - I undertake an awful lot of listed work and there are appliances that can run on 15mm gas pipe.... It's all to do with the quantity of gas available; as the building is listed; you may not need to fit a condensing appliance which all require a 22mm supply within a metre...
There is no reason why a new appliance cannot be fitted (If correctly selected and sized) to the 15mm; then tested under full load to see whether there is enough gas available at the appliance inlet.
I strongly recomend seeking specialist advice from an installer who regularily deals with listed premesis - Or one that's happy to research into them; depending on the heat quantity required and the general condition surrounding the installation; there are sometimes alternative solutions available.
Unfortunately you cannot simply run at half gas, the gas pipework in your property will need to be sized (where possible) to find out exactly what rate of gas it can provide... And that it can meet the maximum demand for the appliance.
Some common solutions I have employed that can be run on 15mm are:
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