The Irish Nationalist Party once got an MP elected in Liverpool:

https://en.m.wikipedia.org/wiki/T._P._O%27Connor

In that case, the objective was to argue for home rule for Ireland. And to look out for the large Irish community in Liverpool.

Welsh expansionism; the Welshmen need Lebensraum.

Joking aside, I also read the title as referring to Plaid Cymru and was very confused.

A lot of people in England has long pleaded with the SNP to stand in English seats rather than just Scotland, because of their other views, so it would not be so strange to see one of the other "regional" parties decide to try to increase their influence by standing throughout the UK.

From what I gather, green policies and opposition to austerity are higher up on the Plaid agenda than Welsh independence right now, so the objective of your hypothetical expansion would pretty much be to duplicate the green party.

Independence for, or expansion into, ancient Northumbria and Mercia perhaps?

https://en.wikipedia.org/wiki/Mercia