BBC have shed some light on the incoming episode -> SPOILERS? (not really).
What was kinda unexpected, however, was this.
As much as comments below this news reveal, the beheading scene was played for laughs there, hence the decision. Since I do not know what really happened and whether the scene was really too direct to show at this time, or they just played safe, I will not comment, but it might be good to remember that avoiding a topic hardly makes it go away.
To sum it up: Robin Hood & Medieval times are a meh topic for me, but I will still watch the episode**, because Gatiss is a good writer and Capaldi is an excellent actor.
*) number one is, of course, this gem of cinematography
**) and whine about it mercilessly