The fact that there's no mention of Wonder 13 being in an Anglo-Saxon area might provide dating evidence for this part of the list. Other areas that contain wonders and which are under Saxon control are listed as such. The area of this wonder was under Saxon control by the time the Historia is estimated to have come together around 830CE (for example, it seems likely that Hereford is the Herefinna listed as part of Saxon Mercia in the Tribal Hidage [See Other Works]). The absence of this detail is therefore strange.

This border area was subject to a variety of fluctuations between Saxon and British ownership*, such that it is hard to give a definitive last-date for British rule, but the area was broadly within the Saxon kingdom of Mercia by a reasonably early stage. The following are some of the key events up to 850 (unless otherwise stated these are from the Anglo-Saxon Chronicles: See Other Works)...

The fact that Bath (Wonder 3) and Droitwich (Wonder 4) are listed in Saxon territory, but this is not, might suggest a date between 577 and 676, though, of course, the absence could be for a number of less significant reasons. The Hwiccae, the tribe under which Bath and Droitwich are given, may have been founded by Penda (c.620: Info), however, Bede uses the tribe when referring to a meeting in ~603, making it hard to give a more concrete earliest date other than 577.