Can I use my Schengen visa for a completely different purpose and entry point?

The visa is not specific to a country or purpose, it's valid for all short stays in the Schengen area. It's also perfectly fine to enter the Schengen area through another country (say connect through an airport in France or Germany) if your main destination is Norway.

Where things get complicated is that if you don't use the visa for its intended purpose, border guards could in principle rule that the conditions for issuing it are no longer met and revoke it. However, if your visa is a circulation visa (multiple-entry visa, valid for more than six months), it should also be completely fine to use it for multiple purposes. Having used it at least once for its main purpose (i.e. going to Norway for work) should help too.

You could maybe take some documentation of past or future trips to Norway with you in case you are questioned (the point being to show that Norway is your usual main destination). In all likelihood, you won't even be asked about it. I am just guessing but I think your profile (permanent UK resident, with a multiple-entry visa from Norway) is not going to raise red flags. If you did not reside in Europe, had a single entry visa and this was your first trip to the Schengen area, border guards would probably be much more suspicious.