Is first authorship a big deal (in terms of highlighting it on CV, etc.)? From August 2015 through August 2016 I was on a Nuclear/Radiation Physics based work placement in a scientific research institute, between my second and third years of my university Maths degree. I am now back at university, half way through my third year.

About half way through my work placement (which I thoroughly enjoyed), my supervisor suggested that the work I was doing could be considered publishable. Being an undergraduate this was (and is) obviously a very exciting prospect. It turned out that we did decide to write a paper, and my supervisor said to be that I would get to be the first author. I don't understand much, but I'm under the impression that's something to be excited about too.

I finished with all the analysis and wrote a draft of the paper. It was then time for me to leave my placement and my (now ex-)supervisor are still in correspondence, whilst he finalizes the paper. There have been various (unrelated) issues which have held him back from spending time on it - but he suspects (as it stands) that he will possibly finish it and submit it to a journal by the end of February. He's basically completely reformatting a lot of the paper, and probably rewriting a whole bunch of it, because being an undergraduate I have no idea how to write a paper. Of course this means that if I do turn out to be first author, my supervisor has probably been very generous.

Long story short - if things go well and the paper is accepted in whichever journal, is this a big deal or nothing to be so bothered about? I ask basically to figure out where this ranks in terms of putting it on a C.V. (or something). Is this something to simply add on as an achievement, or something to really highlight? 

I understand my question is really rather ambiguous. Essentially I'm really not sure what to expect under the circumstances that the paper is published - if anything.
As mentioned here this is field dependent. Given your former advisor offered you the first authorship and assuming that if ordered alphabetically he would be the one to be the first author, then yes, it is something to be excited about.

In my opinion better than highlighting in a CV that you were the first author it would be better to describe what you did in that paper yourself and what was done by the other author(s). And maybe even better that putting this to CV would be to put in to a motivation letter (or any other text accompanying your CV in an application somewhere).