Laluna,
Having watched my wife have 2 NHS births there are some issues not related to the expertise of the midwife and also to the expertise of certain midwives.
Firstly, you are not guaranteed supervision at all when you need it. During my wife's VBAC, the midwife claimed she was not yet in active labour despite regular strong contractions, vomiting and her waters going. This was about 2 hours before she delivered. The obvious reason was that were the notes to have read active labour, she would have needed a midwife with her and constant monitoring. There was a large haemorrhage at the time so there were no rooms or midwives available on the labour ward, so easier to doctor the notes than admit that they failed my wife and put her at risk. Secondly, epidurals are often not really available. They say you may need to wait but, when my wife asked for one with our first son, the wait was 3 hours and, with our second son, she only got a spinal block in theatre when they had to deliver quickly by forceps.
As to the midwives themselves, I am sure you are great but some are very junior and lack basic qualifications. Again, during my first son's birth, my wife became dehydrated but the midwife we were with was not "qualified" to canulate, so she had to wait quite a while to get some basic fluids.
There are sound reasons to go private, although personally I would not be enthused to pay what it costs, as, ultimately, if things become a real emergency, the NHS does come into its own. That is not a criticism of any individual midwife, some of whom are superb and certainly know more about normal delivery than some obs.
We compromised with a private midwife, which was really helpful and costs more like £4k and for that, you get a midwife of your own choosing, contactable 24/7 pre natal checks at home and her there as your "advocate" at the birth.