Partack
So, it's been 7 years,

A quick google shows that this Wales/England boundary looks accurate,  according to a picture of the Severn Estuary found in this PDF by "National Resources Wales"

https://cdn.cyfoethnaturiol.cymru/media/674536/mca-29-severn-estuary-wales-final.pdf

I think it's safe to close this note now based on this information.