Here's what Ian Donnachie has to say:
"The somewhat different experience of George Younger, the Alloa brewer, indicates the problems faced by Scottish brewers in the English and Irish markets when the extension of the tied-house system restricted sales. In the 1860s the firm had established agencies in Newcastle, Stockton, Manchester and London, and had commission agents in Hull, Liverpool, Dublin, Cork, Sligo, Limerick and Londonderry. By 1875, however, the English outlets were presenting problems, and the agencies in London, Manchester and Stockton were closed. Hull was given up in 1891, and Liverpool shortly after. The Irish agencies had all been closed down by 1897, business there no longer being profitable. Only the Newcastle connexion was developed successfully, the solution to the problem of tied houses being overcome by the purchase of two breweries in Sunderland and Chester-le-Street. Apart from the export trade, which probably accounted for about a quarter of the firm's business, Younger enjoyed a large trade throughout Scotland based on a long-established reputation."
"A History of the Brewing Industry in Scotland" by Ian Donnachie, 1998, page 217.
The same seems to have happened to many Scottish brewers. Gradually closing their English and Irish agencies. With one exception: the Northeast. Maybe because the tied house system wasn't as strong there as in the rest of England. Or maybe just because it was close by.
25% is a very large proportion of output to export. I doubt many English breweries, with the possible exception of Bass, exported anything like that much.