Great question, Liz!
A pumpkin is a type of squash, and there are many varieties of pumpkins. I don't know the technical, botanical description of a pumpkin, but they tend to be roundish and orange. I do think there are some varieties that are cream or yellow, though.
There are lots of different kind of squashes, and places like Sainsburies had a good selection: acorn squash, butternut, etc. They are similar, but all have a slightly different flavour or texture. Some are sweeter, nuttier, etc. than others.
I'd have thought that your soup will be just fine with a butternut squash. I think that butternut squash is richer and thicker than pumpkin.
I hope that helps. I have a good squash cookbook (which is in transit with the rest of my household) with hundreds of recipes. If you are interested, as soon as my stuff arrives, I can pass on some to you.
Let me know how your soup turns out! Bon appetite!