Jos: I have looked up a few flags, and it seems that the flag of St David is a yellow cross on a black background and the flag of St Piran (a 5th century Cornish abbot) is a white cross on a black background. So by replacing the blue with black, retaining the red cross of St George on the white cross, and removing the white St Andrew's cross leaving just a red diagonal cross, a flag could be created representing England, Wales, Northern Ireland and Cornwall. Further adjustments could then be made if/when necessary.

This would not represent Wales, as the only changes which you might think would incorporate St David is the inclusion of a black field. This (and the white cross) could just as easily be only to represent Cornwall.
Any way, while Scotland (as a stand-alone) is represented by the saltire, and England similarly by the cross of St George, the established flag of Wales is a red dragon on a field of white and green.
And, the flag of St David is a gold cross on a black background.

ABCD: With regard to what flag will replace the current "Union Jack" ("jack" is really only appropriate for a flag at sea, by the way). A bit of pedantry which fails spectacularly as the flag of the UK can be described as either The Union Flag, or The Union Jack.
https://www.flaginstitute.org/wp/british-flags-2/the-union-jack-or-the-union-flag/The Flag Institute sums up the relevant information.