I'm sure you're all fed up of the threads like this but if at all possible I'd like some entirely honest input, don't be afraid to say it as candidly as you can

Went from being a good student up until university, I got into a top 5 UK university, then my bachelor's went catastrophically If anything I would attribute it to family and financial issues but I don't want to seem like I'm making excuses on my applications as these are vague enough that anyone could claim it. Even during this terrible undergrad, I got consistently high marks in the research modules (All 1st's). Because of this and a recommendation from one of my old supervisors I was able to get into a taught masters program at an equally good university (I figured a change of scenery would do me well) and so far things have actually been going so much better.

I'm nearing the end of the Masters now, just in the final stages of submitting my thesis and it looks like I'm on track for a distinction( 1st class equiv). Seeing all the graduates in physics who have high 1st's and were consistently top of the class struggling with Ph.D.'s makes me wonder about my chances. On top of that, due to the family financial issues I'd mentioned above all 3 summers at university I had to work almost full time and couldn't partake in any internships as most don't pay much if at all so I don't have that to my name either.

My target fields are High-Energy Physics or Astrophysics but I know these are among the most competitive. What are my chances and any advice on how to catch admissions staff's attention despite my poor undergrad

Honestly, I don't think the 2:2 at undergrad is that bad if you have a first from your masters. Your masters is more relevant and should get you an interview. The interview is were a lot of PhD applications are decided and your CV/refernces only need to be good enough to get the interview. A few people have asked this question beforehand and usually they were asked about the 2:2. You will need a good answer to explain why you only got a 2:2 but your masters is vastly more relevant than your undergrad.

Firstly, I would say that there is no such thing as a "top 5 uni".
You either went to Oxbridge which is worthy of mention, or you went to one of the rest., which isn't worth mentioning.
A 2:2 from your uni is the same as a 2:2 from almost anywhere else so you may as well own that straight away.

Secondly, the good news is that your Masters makes your undergrad degree redundant so forget about anything but getting a distinction here.

Thirdly, if you are excelling at experimental work and your PhD is also experimental then you are in a strong position.
If you need some theory for your PhD you should make sure you plug any gaps from undergrad which the masters didn't cover.

I have to say, it sounds like you're in good shape to recover from the 2:2 so good luck.

That's just pm133s personal "opinion", and is quite a ridiculous statement.

One just has to look at the achievments in the history of the UKs top universities. Some are really astonishing. That's not to say other lesser well-ranked universities haven't made breakthroughs.

Hello. Congratulations on your predicted distinction, that's a massive achievement.
I got a 2:1, predicted a 1st but sometimes life throws curve balls and this does affect your education. The University I'm doing my PhD at specify a 1st class honours and a distinction at MSc (this one I did achieve). However, I got an interview for a funded scholarship by emailing the leads directly and asking more about their previous work. I got an invite to sit in on a lecture, a chance to have a look at their offices and got an interview - now I've just submitted.
You can pick up work at your institution, but I started a part time business from my old job (completely opposite to my PhD) to make ends meet, so if I can do it as a single mother of two, you can do it! It's frightening, but it will be worth it in the long term.
Email those you want to work with, ask for opportunities. If they ask about your 2:2 be honest, you had a lot going on and were young, now you're more mature and you learnt from those times, so any curve balls will not knock your performance henceforth.

Good luck