Forgive the brief power level.
For a year and a bit, I studied archaeology at a UK University before dropping out, however during our first year we learned about museum management and how artefacts in museums get preserved and displayed.
Neither of them got anywhere
near the actual Magna Carta because its an 800 year old document. The real one is in an archive, likely on-site, kept away from the public eye because preserving documents that old requires a lot of special preservation methods. This includes adjusting light exposure, humidity, temperature, etc.
In heritage and artefact management there is
the agents of deterioration, which are 10 primary threats that affect all artefacts. One of those 10 includes thieves and vandals, just like those old ladies; I can assure everyone on this forum that the museum they went to went through those agents, took into account the potential for someone to attempt to steal/vandalise the magna carta, and put measures in place to ensure the
real magna carta was at no risk. What they attempted to vandalise was a replica meant for the general public.
To spoil the magic for a moment, if you've been to any museum holding any very significant or important archaeological discovery, and it seems barely protected by anything more than just glass, then unfortunately what you're looking at probably if not certainly isn't the real thing and is a replica commissioned by the museum for the general public, i.e. not archaeologists, as unless you know what to look for its very difficult to distinguish between the real thing and a
very good replica. You can ask to see their archive and see the real thing, but don't expect to be able to unless you're qualified to work with artefacts and/or archives.
So in conclusion not only did Just Stop Oil convince two old women to commit multiple serious crimes, it was all performative bullshit that would have no difference to the actual magna carta. Just like Just Stop Oil, their actions changed nothing and only affected the general public.