I must rectify an error in the title. It was only three words; 'WALES, see England'

First, note this is an answer addressing specifically the encyclopedia reference, not the full history between Wales and England at the time. Despite what I state below, I do think it still works as an example of England's opinion of Wales in 1888, but there are some caveats.

Because: at the time when Volume 24 (URA-ZYM) of the Encyclopædia Britannica was published, there's good reason to think there was no reaction at all to the entry on Wales.

UPDATE: /u/gnorrn sleuthed some out — do check the reply for some 19th century references! The other actual circumstances outlined below still apply.

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This isn't the first such encyclopedic reference; Brewster's Edinburgh Encyclopædia created between 1808 and 1830 includes the same "Wales, see England" line. (As far as I know no modern scholar has ever mentioned this fact.)

The actual entry for England in the Britannica in question reads: "England, comprising, with Wales, the southern portion of the island of Great Britain." The same text appears on a map marked "England & Wales".

Also important is to note the alphabetical order of publication. The England book (volume 9) was published in 1878, a full decade before the Wales book (Volume 24). This means readers already knew for 10 years that Wales was incorporated into the England entry long before the "see England" line appeared. In a practical sense, even though England & Wales was a common form of reference (and used by the Britannica itself), any commentary on Wales would need to wait 10 years if they were written separately.

(Aside: in between the two volumes came the very first time Parliament passed a law that applied specifically to Wales and Wales only, the Sunday Closing Act of 1881, requiring the closing of public houses on Sunday.)

The naming structure of "England & Wales" was long the citing reference in publications even outside of England; from the US, for example, see this publication of the Census office (1907) or this survey of education (1921) or this book on agriculture (1947).

I'm not sure when the Britannica entry first became infamous among scholars. The first reference I know of is from 1963:

In all aspects of public policy, the trend towards assimilation and uniformity was consistent, testimony to the notorious directive in the Encyclopedia Britannica: 'For Wales--see England'.

The fact it misquotes the "For" (like the original title of this very Reddit question) indicates the idea has reached meme status by then. So it's certainly possible it was "notorious" for long before that, although I suspect based on the 10-year gap circumstance the infamy is of more recent vintage.

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Schrijver, F. (2006). Regionalism After Regionalisation: Spain, France and the United Kingdom. Netherlands: Amsterdam University Press.

Morgan, K. (1963). Wales in British Politics 1868-1922. University of Wales Press.