I have speared in all the locations you mentioned, with varying success. As you mentioned safety I would recommend against the Bristol Channel (as it has the second largest tidal range in the World - and I know of 2 diving incidents/"epics" and one climbing epic related to that), by which I mean the wonderful and beautiful South Wales , north Devon and North Cornish coasts (which can offer good surfing, some times, in some places).
Probably safer (but not entirely safe, of course!) to stick to sheltered bays on the English south coast initially (but others might have other knowledge/suggestions, e.g. re. West and/or north Wales coast?).
That all said, I did manage a good day's spearing at Sennen Cove, avoiding the popular but lifeguard limited surfing area.
I think Portland Harbour is often used to get started. As it has a shallow but small beach and is surrounded by a harbour wall and there are usually yachts anchored there (there is a yacht club next to the beach). I've not seen fish there but there are stories of bass, scallops and oysters, possibly flat fish (?); perhaps nearer the entrance of "the Fleet", the lagoon that meets it.