At least the ones that match are candidates for clearing the name without loss of information.
But there are errors too, I had noted w.1425467008 (and 3 adjacent) where one field has BN1 2xx and the other BN2 1xx.
And setting your query to "!=" I see w.1432593337 with a random typo/mismatch. 
But there are also shape errors, I noted one of the final ones (added *after* your discussion had concluded!) at the N of Peacehaven,
 w. 1436971553 managed to get both 10 Telscombe Road and 10 Greenacre - but at least this sort of error suggests they might be deriving the areas themselves not copying directly from a copyright source.
I can see where one could get the areas where a small road has 1 postcode, or 2 for odds vs evens, but not sure how they got (or how we can verify) where a longer road has a change midway eg w. 1416829146 BN2 4TE starts from no.96, and 4TF up to no.94.
But there is probably no big hurry, if we're happy to change these tags by very careful overpass+josm edits rather than reverting.
A bunch of unwanted St->Saint edits are getting gradually edited back.
There's some bad 3D work but the worst can get fixed up.  Some arbitrary/random? colour, height values though, and a general lack of care.
And there are also some type=site relations by the user which are just grouping stuff in categories and can go at some point.
Cebderby (Clive)