Pressure groups tend towards a single focus.
That all depends what you mean by a focus. Greenpeace, for example, has a single over-arching principle, but it is expressed by many different campaigns over time on specific issues. Similarly churches normally have some sort of overall principle, and it is expressed via different campaigns on specific issues. It seems a good parallel to me.

Of course, there are also pressure groups like 'Save Lewisham Hospital' where there is a single goal and essentially only one campaign, but by no means all are like this.

Would I call the Conservative Party a pressure group? No, but that is because a pressure group is trying to put pressure on someone, normally the government, and that can't apply if they are the government. Would I call whoever is in opposition at any particular time a pressure group? Probably not, but I'm not certain. Would I call the Anti-European wing of the Conservatives a pressure group? Yes, I would.