Stonehenge altar stone from Scotland

Well the spelling is easy to explain; it was really only in the last 200 years that the spelling of names became mostly standardised.
Apart from that what point do you have about Boudicca?
Most evidence about named people is written evidence. 
However, no historian would accept the existence of a named person from just one written source, they'd want to find multiple, hopefully independent, written sources.  
If there is then a inscription in stone somewhere, a gravestone for example, that is also independent of the written sources, then you are reasonably certain a named person existed.

From the archaeological record archaeologists make analyses.
Historians then take those analyses and see if they fit within the established historical reference.
Example:
If you find bones that have hundreds of small striation marks all along the length of the bone there is really only one conclusion to come to - the flesh was stripped from the bones.
There is then the question of why?
There are some burial practices that require the stripping of flesh from bones; but I don't recall them being present in the British Isles.
That would tend to mean the meat was stripped from the bone for consumption.
What happens then if the bones found so treated are human bones?
If there is a written historical reference that Human Sacrifice / Cannibalism was practised then the archaeological record matches the historical record - they are different things.

Note: This this is an example, nothing more.
  

All The Best

It was always both, its just one was preferred over the other for a long time, then that changed.


All The Best

It didn't come from Scotland, Scotland didn't exist then

What we can see today is basically a Victorian tourist attraction. Nobody has a clue what it's original purpose was because it is 3,000 years old. Also, it is likely that the stones were repurposed repeatedly to serve the people of the time.