Having done very many crossings of the pond, I would consider that the interviewee's answer is the best I've read so far. No O2, = not going far!
(of course I'm assuming that it's the cockpit gaseous oxygen that has depleted, as the SLF O2 is chemically generated). Upon noticing that you have little or no cockpit O2 left, it's down to 10,000 or MSA if higher,(it is'nt on the NYC-LHR route unless if over Greenland), and, yes, fuel burn will be much higher, thus it is unlikely that you will make landfall eastbound at this lower altitude. Upon consideration, this was a clever interview question. The answers from candidates would be expected to be commensurate with experience. Don't know your experience level, but in any case, your answer was SAFE.
Hope this helps, and good luck with those interviews
