Are you intending to visit Salisbury Cathedral soon? If so, it is worth checking if there will be a chance for you to see what you want to there.
There was an attack today on the Magna Carta on display there. One of only four originals it was attacked with a hammer. Due to previous security devices only the case was damaged.
I don't know how that event will alter availability to visit the Cathedral in the short term nor seeing the Magna Carta.
That's why I recommend checking before visiting.