Hello Clive,
Technically LPG always stands for Liquefied Petroleum Gas. In reality it can be any petroleum gas that can be liquefied, by increasing its vapour pressure or cooling it.
Virtually pure propane does exist as a 'test gas' used by appliance manufactures to test an appliance for performance and compliance to the relevant standards. Similarly there are purer versions of most LPG's for the same reasons but these are very expensive, and even your pocket would suffer if you insisted on only these for your caravan.
Commercial gasses such as those sold by Calor have to conform to some standards but are rarely pure. They usually contain varying smaller proportions of other Hydrocarbon gasses. This is either by design such as mixing of some Propane with butane to help with lower temperature performance, or through the coarser and thus cheaper fractionising process that makes the gas. Incidentally all LPG's are colourless and odourless. The manufactures always add a stenching agent so the gas become detectable by smell.
As for the mix of commercial propane across Europe, there are international standards for particular grades of gas, but as with petrol there can be variations within that standard and between manufactures. Some national standards vary as well, so rather than a gradual change of the mix supplied across to Spain, it is more likely to change at boarders, and with the supplying company.
Not directly related to LPG but may be of parallel interest; There are significant differences in the mix of natural gas supplied to homes, so much so that manufactures of appliances wishing to supply across Europe have to arrange for special supplies of test gasses to be able to test their appliances. In fact some manufactures find they still have make small changes to appliances to make them work and therefore saleable for some EU countries.
To demonstrate how variable LPG supplies can be: look inside your caravan refrigerator. You should find that it has a printed label, with a list the countries and the relevant gas categories that apply.