Hi
I dont know about Lydney itself but we live further down the Severn Estuary: there is a very strong tide here as you know, the Severn has the 2nd highest tidal range in the world meaning that the river/sea goes out a long way at low tide. There is a lot of mud in the Severn (clay) so I don't allow my dogs to range too far out as there are very soft patches but on the 'beach' or close to the coast you'll probably find fairly firm ground. Due to flood risk much of the coast is protected by sea defences and the walls and earth banks provide good walking for the dogs.
If you are planning it for a holiday just make sure that you have the facility to wash the dogs. The mud is like clay 'slip' and gets everywhere - particularly if they charge about like loonies, as mine do! - I even have to wash it out of their ears :) It doesn't smell great either but the dogs don't care
I found a YouTube video
here which looks like the sort of beach I recognise - beautiful in it's otherworldly way but muddy!!