Why did Britain care that Germany invaded Poland in 1939?
Powerful capitalist and state interests in Britain had a rivalry with Germany for markets, power and influence. They had ganged up with the French to try and keep Germany down in the First World War, but German expansionism and rearmament threatened that post first world war settlement and conflicted with their interests.
They didn't give a damn about Poland, or that Germany had been taken over by a bunch of genocidal fascists. In fact, leading figures in British industry and the aristocracy, including the guy who was briefly king in 1936, were actually quite keen on the Nazis and how they locked up trade unionists and persecuted Jews, and the Nazis had, of course, nicked the idea of concentration camps from how Britain ruled South Africa and India.
It was just that Britain and France thought, well, ruling the world was Britain and France's job, and Germany sort of muscling in on the act was a bit of a threat to their comfortable position lording it over everybody. Of course, the war bankrupted Britain and France and demolished Germany, and the US then took over the role of running and exploiting the rest of the world from the three of them.
That post second world war settlement is starting to crumble a bit now, so look out, there will be warmongers about. You think Brexit is bad? You ain't seen nothing yet.
Put short: it didn't. Had it cared at all, it would have done something more than just declare war on Germany.
Poland and England were in mutual military alliance which - in theory - was supposed to prevent German expansion (much like the Polish-French alliance). Too late did the allies realize, that despite of the signed agreements Germany managed to rebuild their armed forces, which - in many ways - exceeded their military potential.
But neither England nor France was willing to provide actual help. Both countries declared war on Germany and... that's practically all they did, in hopes that German expansion will continue East, probably not realizing existence of the Ribbentrop-Molotov Pact.
The thing is they didint, where did you get information that they did?
Poland and Britain had a pact where it is said that if Germany attacks Poland then Britain will declare war and launch offensive within 14 days from time of mobilization. They didint fullfill this treaty which is known as western betreyal (together with Czechoslovakian situation).
The whole allies idea was around the belief of controlling appleasament towards Hitler, imagination that allies had control over Hitler and that Hitler would never dear to attack France-Britain, even maybe Hitler would eliminate Russia which would be perfect scenario for the allies, at least from their point of view within their view of the world.
Nazi Germany had repeatedly violated the Treaty of Versailles(Anschluss, Rhineland remilitarization, Munich Crisis, rearmament, construction of aircraft and U-boats, etc), and was clearly on a war footing in the late 30s. After the Munich Crisis, Britain was given a black eye when German troops marched into Czechoslovakia despite promising not to, and Britain guaranteed Poland's independence in an attempt to stop further Nazi aggression.
If a war seems inevitable, why let your enemy keep conquering territory and getting stronger before stepping in? Britain desired French hegemony on the continent, and that was no longer the case by 1939.
Britain did not care.
Poland was a new state, established after WW1, and part of the post ww1 settlement and system that Britain established after the Great War. Germany retaking their territory and destroying the polish state would the final dismantlement of that system. Britain had also guaranteed (with France) for Polands borders. The had to declare war, especially after chickening out during the German conquest of Czechkoslovakia.