Does anyone know what UK cities make up the so-called 'Seven Triangle'?

Question

I think Bristol is included.

Answer 1

I'm guessing that you are actually looking for cities along the lower estuary* of the River S e v e r n. This body of water is also called the Bristol Channel.

Cities in the area include:
Cardiff, Newport, and Bristol.

Hope that's what you're looking for.

* an estuary--or sunken valley forming the mouth of a river--is sometimes inaccurately called a delta, as in China's Pearl River Delta, near Hong Kong. Other famous estuaries include Chesapeake "Bay" and "Río" de la Plata (Silver "River") between Argentina and Uruguay.