I am so sorry to hear this- what a horrible shock for you and your family.
Please try and get it all in writing- if you intend to take this further you will need to have everything logged.
I suggest that you write to the practice manager- or head vet and list everything that happened (as you understand it) keep a copy of the letter for yourself.
See what the reply is and take it from there.
If you are going to make a formal complaint to RCVS then you will need all this paperwork.
It's a miserable process to go through, especially when you are in shock and grieving, but a lot of the time nothing gets done because people think about this when it's too late to do anything.