As interesting as this seems to be, there seems to be not much light available to shine onto this issue, except for the fact that such a man existed, although probably not really in command of that late and lonely mission:
Among eight Mongol prisoners captured in Austria during this reconnaissance there was an Englishman. He had once been a Templar, but after being banished from England for an unknown crime, he had travelled through the Middle East and entered the Mongol service as an interpreter. It was said that he spoke seven languages. Matthew Paris mistakenly records that it was this Englishman who delivered the letter to King Bela which was in fact intercepted by Friar Julian, and some say that he was in command of the reconnaissance force, but this is unlikely since such a command would not have been given to anyone other than a Mongol. The only certainty seems to be that among the many nationalities in the Mongol corps of interpreters there was at least one mysterious Englishman.
— James Chambers: "The Devil’s Horsemen. The Mongol Invasion
of Europe", Book Club: London, Edinburgh, 1979, p 110–11.
If there ever was to put a name on it, or him, shaky evidence points to an almost legendary figure, note the surname, of Robert Eracles (Hercules):
Tartar Khan's Englishman (i.e. the book mentioned in Twelfth's answer) [speculates] that the Englishman had probably been Master Robert Eracles––an English knight and former advisor to King John who had been exiled and eventually picked up by Mongol talent scouts and taken to Mongolia.
— Tim Cope: "On the Trail of Genghis Khan: An Epic Journey Through the Land of the Nomads", A&C Black, 2013, p474.
Master Robert Eracles was an English knight who was present at Runnymede for the signing of the Magna Carta. He thereafter journeyed from the Middle East to Mongolia in 1243, whereupon Genghis Khan enlisted the linguistically talented Eracles as a diplomat. The knight turned Long Rider then rode from Central Asia back to Europe, where he was beheaded by Europeans for having assisted the Mongols.
— Historical Long Riders
Both these accounts are not hard evidence and depend on the overly optimistic reconstruction made by Ronay. This is deemed not impossible but "conceivable".
Cf. — Felicitas Schmieder: "Europa und die Fremden. Die Mongolen im Urteil des Abendlandes vom 13. bis in das 15. Jahrhundert", Jan Thorbecke: Stuttgart, 1994, p54. (worldcat/academia.edu)