As has already been said, 'Thaxted' was so named because that is where Holst was living when he wrote it. I don't know of any evidence that he based it on an existing tune; though naturally he was familiar with traditional idiom, having -for example- set songs collected by Dr George Gardiner quite early in his career.

A few songs were collected in Thaxted by Clive Carey and George Chambers between 1911 and 1921, all from a Mrs Hollingsworth. When I have time, I will check those I have access to to see if there is any connection; there is always the possibility that received wisdom is not accurate.

RVW noted several songs in Herongate, from several singers; which one is meant here I don't know. What was the hymn set to it?