Update
The section south of Gibralter point is shown on the OS online Map as open
Unfortunately this is not true. It is planned to be open late Autumn

Most of it is open
Apart from sections which are officially open there is much of the South West Coast Path and Norfolk Coast path as well as all rights of way on the coast.

Frustratingly there are a few sections currently impossible

Lincolnshire: Gibralter Point south for about 3 miles. This is offically open but cannot be used! A bridge is missing and the landowner is not positive about allowing a nearby bridge to be used.

Suffolk: Woodbridge to Bawdsey Manor and also around Great Oakley

Essex: Colchester to Salcott, but you can get round Mersea Island.
Section from Jaywick round to Colchester but some is right of way
Bits between Burnham on Crouch and South Fambridge, some rights of way are on coast but other bits not possible

Hampshire: Lymington to St Leonards Grange, a new section wil replace Solent Way. Also much of Isle of Wight

Somerset: Cleveland to Sand Bay

S Cumbria: Grange to Barrow in Furness

I think that is it. These bit should open this year but I am not optimistic