Yes the HA does do Barbel very low down the River, the Royalty is proof of that, but the Severn is a different kettle of fish, and I am very sceptical about anyBarbel below Gloucester.
However I would welcome any evidence to prove that they do exist in a Estuary such as the Severn.
Strange timing this as I am in the process of looking into Lower Lower Severn possibilities, but to put it into perspective I was speaking with the current Severn record holder last Friday, and he is currently on his 38th Lwr Severn barbel blank, he has done 80 odd!! and that is no lower than Tewkesbury, good dedicated angler to he is, my thinking is if he struggling then what hope is there for the rest?
Of course there are particular areas on the Lower that do contain Barbel, but any local would know that and would have no need to ask!