The flat sides and rectangular water-filler hole identifies it as a Hawksworth tender. This type of tender (except for those fitted to the 1000 Class Counties, which were wider) was interchangeable with the earlier 4000-gallon Collett design and could be seen running with Halls, Modified Halls, and Castles, having been swapped during servicing. However, the Kings, as a matter of prestige, each had their own dedicated (Collett) tenders and as far as I know no King ever ran with a Hawksworth tender. So no model was ever manufactured like this and the unprototypical pairing seen in your purchase must have been the work of a previous owner, perhaps by accident. Rather than look for a GWR King body to "match" the GWR tender, I'd dispose of the tender in favour of one more appropriate for a King. Incidentally, the Dapol County and Castle on which the Hornby models were originally based had Hawksworth tenders whose width (OO scale 8' 3") was a compromise between the County (8' 6") and Castle/Modified Hall (8' 0") types. I don't know about the more recent Hornby models.