I cannot speak for the English but I know the French wanted a strong Polish ally as a replacement for Russia (given the revolution) as an eastern deterrent against Germany. However one of the most contentious portions of Versailles was the Polish Corridor, whose main purpose was to give the new Polish state access to the sea. What I am wondering, is couldn't the French have killed two birds with one stone by advocating both for a united Polish-Lithuanian State whose port would be the Lithuanian coastline?
Let me know if this is better suited for r/HistoryWhatIf, but I am genuinely curious why the Allies didn't take the opportunity to make such a large state in Eastern Europe as a bulwark both against the rise of communism in the area as well as to keep tabs on German eastern-ambition.
You're going to want to look up Intermarium, the vision of the first leader of independent Poland, Józef Piłsudski. Intermarium (literally "Between the Seas") was the idea that there should be a Polish-led loosely federated state spanning the entirety of Eastern Europe between where Germany ends and Russia/the Soviet Union begins, including Poland, most of what's now Belarus and Ukraine, and Lithuania (and maybe Latvia too, depending on who you ask). There were numerous reasons to think that this dream was plausible in 1918 or 1919: Ukraine, a gigantic, really valuable piece of land, was in a massively confusing and convoluted free for all civil war between quite a few different factions and Soviet domination of it was not yet established or at all certain to occur, and most of the rest of Eastern Europe was newly independent so the Baltics aligning with Poland did not seem all that far fetched either.
Several things make Pilsudski's dream of Intermarium just that, a dream. First and most importantly, Soviet Russia pulled itself together. Organizing all of Eastern Europe under Polish guidance could only happen during the interval when Germany and Russia were both out of contention, and Russia reemerging under new management as a great power again changed the equation dramatically. Instead of dreaming of conquering Ukraine, Poland instead found itself on the defensive in the Polish-Soviet War of 1920-1921. While Poland would successfully win the war, it was only by the skin of its teeth, and Poland's eventual deal with Soviet Russia in the Treaty of Riga ensured that Poland's ambitions towards Ukraine were ended.
Secondly, Poland's borders were finalized with the new state of Lithuania in a highly controversial manner, putting the Lithuanian capital of Vilnius/Vilna/Wilno, after a...highly irregular and confusing Polish "invasion" of Lithuania that was unofficial but secretly approved. The rump Lithuania, deprived of its historical capital, proved deeply hostile towards Poland in the aftermath of this, so even though Poland did add the historical capital of Lithuania to its territory, it ended any hopes of a diplomatic federation with Lithuania in the process.