There are almost as many definitions of "nationalism" as there are nations, but in order to attempt to answer the above question, some working parameters of both nationalism and Unionism obviously need to be set up- this one from M Crawford Young of the UCLA International Institute is as good as any I’ve seen:
"Nationalism I would define as an ideology claiming that a given human population has a natural solidarity based on shared history and a common destiny. This collective identity as a historically constituted people crucially entails the right to constitute an independent or autonomous political community. The idea of nationalism takes form historically in tandem with the doctrine of popular sovereignty: that the ultimate source of authority lies in the people, not the ruler or government. The foregoing definition of nationalism will be found in any classic text with minor variations."
Key there, I think is that "natural solidarity" which arises from a shared history and common destiny.
In my opinion, UK (as opposed to solely Ulster) Unionism arises from five main sources:
By definition the UK Unionist must be a multi- not a mono-culturalist, the basis of his nation has at least four different, core cultures. Many believe in the concept of Unionism because they wish this diversity to continue under a single nation-state’s "umbrella"
The UK is a secular, liberal and progressive democracy. There are UK Unionists (particularly in Northern Ireland and Scotland) who believe that those qualities are best safe-guarded by policies on such topics as abortion and gay-rights being determined by one sovereign parliament at Westminster.
If you want to be cynical you could ask why Brown, Straw, Hain and many others within the Labour party are now suddenly committed Unionists? Solely political reasons. Looking at the topic more positively; the United Kingdom as a collective is a recognised world political power with the advantages that position brings. Divided, that power in such arenas as the European Union and the United Nations would be greatly diminished. Those who would not consider themselves as "cultural" Unionists would tend to follow this line.
Presently applies more for Northern Ireland, Scotland and Wales than perhaps England (although the inconvenient fact that is sometimes forgotten is that all three countries have contributed by suppling the pool of labour and knowledge which has led to the UK’s position in the World economy today). UK Unionism believes that, in the absence of any credible alternative in terms of the financing of such things as health provision and education, economically the Union still makes sense. Many non-cultural, non-political, maybe even non-voting, unionists (with a small "u")see this argument as the prime motivation for the United Kingdom remaining a unitary state and economy.
5 British nationalism:
Finally, yes, there is an element which derives their Unionism mainly from their belief that the people of the UK have a “natural solidarity based on shared history and a common destiny”.
But it’s only an element.
A Unionist is someone who believes that the Union of England, Northern Ireland, Scotland and Wales should continue, nothing more nothing less, and as I’ve shown there are different reasons as to why that belief arises. The fact that Unionism is not a homogeneous political movement appears to be a concept that many Celtic, and in particular Irish, nationalists, find hard to grasp, but different people from different parts of the UK will have widely varying reasons for believing in a United Kingdom. That being the case, then the answer to the question I posed at the beginning, therefore, is that whilst a British nationalist must consequently also be a Unionist, a Unionist need not necessarily subscribe to the principles of British nationalism.