The idea that you 'need' a very well insulated property isn't really true, particularly if we're comparing against an old oil system. What you really need is well designed system, of which insulation is one component. Insulation makes it cheaper to run, but that's the same for any fuel source. In older properties it's usually the drafts that are the biggest problem, and these are pretty easy to deal with, if a little time consuming. Still, it shouldn't be too difficult to design a heat pump install to outperform an old oil setup. A heat pump will be several orders of magnitude more efficient than an oil boiler (say 85% vs several hundred percent) and heating oil is not too cheap these days. The main stumbling block in achieving that efficiency in older properties is usually the cost/hassle of the radiators/pipework/etc to make it viable. It's a full system overhaul rather than a straight swap.
What you need to make a heat pump viable is a very low average flow temperature (in the 40s). I say average because you also need weather compensation so that you are able to maximise efficiency when it's warmer, which makes up for the higher cost when it's very cold - with weather compensation, the flow temp varies according to the outside temperature (heat pumps get more efficient at lower flow temperatures). But, lower flow temps also require larger radiators in order to get the same amount of heat into the room (which is why underfloor heating is great). Realistically, if you're going from an old oil install to a heat pump then you're probably going to need every single radiator to be at least twice the size (that can mean swapping to a double or triple panel, rather than doubling the physical area). Radiator sizing in older properties can be all over the place though, so you won't really know until you have a thorough heat loss survey done.
Which brings me back to point number 1 - the design of the system. As is always the case when free gov money is on offer, the chancers and cowboys move in to take advantage. You need an experienced, reputable company to do the work, and it's probably not going to be a case of getting several quotes and going with the cheapest because it's a complex process, and the design/commissioning of the installation will make or break it.
It's also worth mentioning that heat pumps are ideal for retired people who are home all day, as the system should be designed pretty much constantly in order to keep the flow temp low. That is obviously the opposite of a traditional oil setup, which is on for a couple of hours in the morning and a couple more in the evening.
