How much does an undergraduate degree matter for an academic career? I'm a PhD student, in medical physics at a very good collegiate university in the UK. I did a four-year undergraduate masters' (in physics), and I'm just finishing a four year PhD. I'm now at the awkward stage of applying for grants, junior research fellowships and postdoctoral fellowships -- and I'm experiencing an awful lot of rejection letters, often after being shortlisted and interviewed. 

My question is this: how much does my undergraduate degree actually matter at this point in my career? I wrote two articles (on computational biology) as an undergraduate  that were published in  good (impact factor ~5) journals, and by now my publications list has eight items on it (excluding conference proceedings), including a PNAAS article (albeit not first author) and several articles in the main journal in my field. I've won prizes, lectured, and got a teaching qualification. Yet I still keep being rejected for positions that are 'appropriate' for me to apply for.

When I speak to older (successful) colleagues about their experiences, they often drop things like "Of course, coming first in the year at [Cambridge/Oxford] helped me get my Junior Research Fellowship, and even the Tutorial Fellowship later" into conversation, and the vast majority have a very good degree. I didn't do fantastically in my undergrad degree -- I narrowly missed out on a first class degree (69.96%), largely due to one bad exam. I really can't help but think that the reason I'm finding it so hard to get funding is because I didn't come first in my year -- but I'm up against people who presumably have successful publication histories and did. 

If there are a large group of equivalent, good, candidates for one position, do funding bodies and interviewing committees look at what's different between everyone? Is the fact that I'm objectively a second-class physicist holding me back? If so, what can I do about it? Or is it the case that these funding bodies do some sort of crazy weighted sum, whereby one-tenth-of-a-nature-paper is equivalent to being first-in-year? How much of a hinderance is it being -- as I was -- in the top 20% of your year, as opposed to the top 10%? Does the importance of your first degree erode over time?

I realise that I should feel pleased to be shortlisted where -- to give an example -- 283 people apply for one position, and I was in the final six. Yet 'feeling pleased' won't pay the rent next year, and I'm really starting to despair. Should I accept that this limitation is always going to hold me back in my chosen career path, and therefore just go and change it? 

Aside: I'm also concerned that, being an interdisciplinary person -- an MRI physicist -- I'm going to come across as being "too medical" for a physics position, and "too physical" for a post in a biochemistry department. In practice, my research ranges from Schrödinger equations to talking to cardiologists, and I believe that either location would be appropriate. This, however, is a whole other kettle of fish!
I will give some answers from an American point of view (in math), but probably this is widely applicable.  The answer in short is that your undergrad degree means essentially nothing at this point.  It's only role was to get you where you are now.  

Now, for some of your secondary questions. 


  If there are a large group of equivalent, good, candidates for one position, do funding bodies and interviewing committees look at what's different between everyone?


Yes, but your undergrad degree is weighted almost nothing in these considerations.  About the only case where it can help is if someone on the committee knows you from undergrad and has a positive impression from then.  The primary consideration is your research (both how good it is and how good of a fit it is) and expert opinions of your work (e.g., recommendation letters).  Secondary considerations may be where you've published, how active you are in giving talks/going to conferences, impressions of committee members who know you, previous awards you've gotten, and other professional activities.  (I'm assuming the things you're applying to don't involve any teaching.)  Once you've gotten to the interview stage, other things like how good you are at communicating, how enthusiastic you are and how you get along with others come into play, but no one will care about your undergrad degree there.


  Is the fact that I'm objectively a second-class physicist holding me back?


If people think you're objectively second-class, it's not because of your undergrad.  Maybe you are, maybe you're not, but it's all about your research and maturity now.  


  Should I accept that this limitation is always going to hold me back in my chosen career path, and therefore just go and change it? 


Only if you want to.  Right now, funding is scarce, there's a lot of competition, and it is hard to get fellowships/grants.  The difficulty varies by situation--some fields have more opportunities than others, and for people who are in between established fields, like you may be, it can take time to find your place.  Almost everyone has to struggle, and it's easy to get discouraged.  It's how you decide to deal with it that's important.  If you want to fight for your place in academia, then do it--your undergrad is not holding you back.  If you want to look for an "easier" career, that's your choice.

PS If you're worried about how fit you are for academia, and it sounds like you are, you should talk to your advisors/mentors.