Restricted zones on charts.
 

The Severn Estuary has three circled area's, I presume, that have fly over restrictions.
EGR 154 is a nuclear power station and EGR 155 a decommissioned one.
The next one, going inland, is twice as big, and joined to the other two.
It has the designation EGW SEVERN.
Without breaching the official secret's act, which I have signed, just what do the three letter ident's mean and why is EGW SEVERN seemingly restricted?

"EG" means it's within the UK FIR. "R" means restricted area.

I think the third area you mentioned is the Severn Estuary bird sanctuary.

Much obliged, Shy Torque, for your expeditious reply. Appreciated. The bird sanctuary, for migrating birds, et al, is at Slimbridge, on the south side of the Severn Estuary.

Yes, I'm very much aware where Slimbridge is, the marked circle goes up to 4,000 feet. If that's not what you referred to, I don't know of any airspace that is restricted (Slimbridge isn't actually restricted). Where did you find your information?

The clue is in the colour. Lime green always indicates a bird sanctuary.

The AIP confirms that it's centred on N51:44:29 W002:24:05, which is WWT Slimbridge, with the note "Pilots are requested to avoid the area from mid-September to early April".