For evidence, start with this:
www.healthcareforlondon.nhs.uk/assets/Publications/A-Framework-for-Action/maternityand NewbornReport.pdf
Note the recommendation that obstetrics units have 98hr+ per week consultant presence. That requires a reasonably large unit to be viable.
The evidence is clear across lots of healthcare: the higher the throughput for complex procedures (ie what obstetricians are supposed to catch), the better the care. Read what Phil Hammond (MD in Private Eye and the hero who broke the Bristol hearts scandal) has had to say about the appalling intransigence of many clinicians to stop doing procedures where they know that throughput isn't high enough for safety.
I'm a huge fan of personalised care we had our first baby in a birth unit and our second at home. I recognise the risks of depersonalised care in a big obstetrics unit. But I also recognise the risk of unnecessarily poor outcomes where things are going wrong and consultants aren't available. Of course, travel time is an issue as you say but there's no point travelling to the closest hospital if it doesn't have a consultant to hand when you get there. (And incidentally, travel time is important but not overwhelmingly important, else we'd have to build about a gazillion hospitals all over the countryside)
Perhaps I'm out of line, and the Whittington is providing 168 hours of consultant cover for the 3000 births. Do you know? If that is the case, then there is ultimately an issue of whether it's right to spend that much on those births vs say shifting a tranche of births to other locations that already provide the consultant cover and also opening up more standalone birth centres or increasing the size of home birth midwifery teams. The budget is not unlimited, after all.
It's a really good question as to how the Royal Free, UCLH and other hospitals eg N Mids will cope with the extra births. I guess the answer depends on a few things, eg how close to capacity they are at present, what room there is to expand the capacity, how many births can be (and should be) managed in more appropriate settings eg home births etc.
Finally, on staff retention, there are lots of factors that play into staff retention including the size of the organisation (with some preferring the backup available in larger, others preferring the intimacy of smaller) -- but I think there will be other factors higher up the list. For example, Hospital A is renowned for autocratic consultants who treat the midwives like silly girls; Hospital B is known for having instituted true multidisciplinary teams. Unsurprisingly, it's B that has the better retention.