Let me quell the myth that oil combis are not as good as gas combis. Most of them are better. Look at the Firebird range and outdoor models of their boilers are available. However you might like to consider a heat pump which subject to meeting various conditions can attract Renewable Heat Initiative (RHI) payments. Bear in mind heat pumps produce lower temperature hot water to circulate through the radiators so require greater surface areas of emitters. Both oil or heat pump sources can work with an unvented hot water cylinder. If you go to a heat pump once a week or so the temperature of the hot water cylinder needs boosting with a standard immersion heater to avoid risk of legionella.
If the old airing cupboard is cold it's easy to add a small radiator to compensate for the loss of the heat from the old cylinder.