My vote goes to two possible candidates, who may be the same person. The first is Ismail Jabbar, aka Abdul Jabbar, aka Abu Maryam Muhajir, who used the Twitter handle @Jabz_26. (The current user of that handle appears to be someone different.) More on him here and here.

Also check out this video of 15 fighters from Sayfullah Shishani’s jamat (just talking, no violence or graphic images). Sayfullah Shishani was killed in the battle against Aleppo prison, and there was a new emir, who also got killed shortly thereafter. Then came this video with the announcement of a new Emir, Abu Ubaydah al-Madani, who was formerly the group's Sharia scholar. The fighters take turns speaking, and all but one speak in Russian. The last fighter to speak uses English and says he's British. He's in the second row on the far right, and begins speaking at 9 min.19 seconds in.

Not only does he resemble the masked executioner, his voice and accent are similar. According to Chechens in Syria, it may or may not be Abu Maryam. He says his name, but I couldn't really make it out. It sounds like Abdul Ramin. He says they will establish Sharia law, and then:

“I represent from Britain here in the UK... we will fight the mushrikin, the murtadin, we will fight Bashar, we will fight the whole world until [?] Allah is the highest.”

So while it's far from certain Ismail Jabbar (Abu Maryam) is the same guy in the video with the Chechen fighters, they both resemble the executioner. (Jabbar appears to be left handed, as does the fighter in the video, who holds his gun in his left hand.)

I think the person in black in the two videos is the same -- but that he is not the actual executioner, which would account for authorities chasing at least two people. Also, there are reports that up to 12 arrests may be made, as authorities are seeking those who provided money and travel assistance to the killer(s.)

Is the killer in black one of the English speaking guards who tortured the kidnapped journalists? Hard to say. A lot of the British recruits ended up in Aleppo or Raqqa where the journalists were held, having trained with the Chechens, some of the toughest fighters. But some are really new to Syria, having only arrived there in mid to late 2013. Some of the kidnapped journalists now speaking out were released before some of these recruits even arrived, let alone finished training.