AHC: Earliest possible rise of Nationalist Parties in the UK

Do what is says on the tin.

Have nationalist parties like the SNP and Plaid Cymru enter the political mainstream much earlier than in OTL. Bonus points if you get devolution on the table before or around WWII.

The Scottish Labour party, which was created in 1888 initially wanted devolution. Get Labour in power for longer after the First World War and you could see it happening in Scotland and possibly Wales, then Nationalists would naturally do better.

Anything else?

Not entirely sure about the plausibility but maybe separatist groups could rise during the early years of Queen Victoria's mourning. Republicanism did have a boost during this period so maybe the same could happen for nationalist groups etc. Allow for some minor divergence's during the Great War, worsening the effect of the conflict. This can allow for the otl Irish Civil War to spread include Scottish, Welsh and possibly Cornish nationalists. If this is successful to a noticeable extent, the remainder may develop into a somewhat enlarged English state. Again not sure of the plausibility but I do think with the proper divergence during the Early Victorian era this would cater for a future somewhat like this.

A bit far fetched, but Joseph Chamberlain succeeds in some semblance of Imperial Federation in the late 19th century. Devolution of sorts to Wales, Scotland, Ireland, England, Canada, Australia, New Zealand, the Cape, etc.

Leads to soft nationalist parties in each devolved country.

Unlikely, I agree, but certainly not beyond the realms of possibility...

AFAIK, Home Rule for Ireland and Scotland was and issue on the cards prior to the Great War. If you can find a way to butterfly, or more likely delay, WWI, you might be able to get devolution by the 1920s.