Which historians are you reffering to that said that Poland charged cavarly with tanks?
It was said by an Italian correspondant named Indro Montanelli that the Polish charged German Panzers using Lancers and horses. German commanders who were present at the battle also confirmed cavalry charges by the Polish.
I'd suggest you look at the numbers of the british army in 1941-1942 and how well they actually performed. What are these so "modern weapons" you speak of?
I did look at the numbers and my research tells me the British Army was 860 thousand to 900 thousand strong in 39 compared to the Polish who had 940 thousand approximately. Did Poland have numerical superiority? Sure(Not by much though) but the Invasion of Poland proved that their tactics, equipment and troops were of low quality. This is supported by the fact that casualties for the polish during the Battle of Poland was 900 thousand(Lol!), compared to the British who lost around 300 thousand in total(Including civilians). This is early on in the war mind you, once the British Empire was operating at full capability, the amount of troops, weapons, ships and etc were increased tenfold.
As for modern weapons, do I need to remind you of the Lee Enfield and the 17 Pounder? The Lee Enfield was modern because it was still the best infantry rifle in WW2; the Enfield was the weapon of choice for Colonial troops in the Boer Wars, conflicts in the Middle East and WW1 and WW2; it was tried and true.
The 17 Pounder was also the only AT gun that could take on Germany's heaviest tanks; what did Poland make that rivaled the 17 Pounder? Nothing.
I'm interested in why exactly the british troops were well trained, had capable officers and extremelly well equipped yet still got horribly defeated in France, Japan and almost lost Africa.
You can't win every battle. None the less, the British Army were constantly winning battles in 1941 and after, and often against enemies who had numerical superiority. Let's not forget though that the British Empire's bread and butter was the Royal Navy; it's the main weapon that established battlefield superiority, but the British Army weren't incompetent idiots and ill-equipped. Quite the contrary actually since they also had an extremely good Army on top of the World's greatest Naval force.
Somehow the british empire consisting of india, canada and the british isles losing a bunch of battles againts germany, japan and what not makes them very well trained and experienced.
Again, you can't win every battle. The British Empire was decisively winning from 1941 and onward.
Yet a much smaller Poland fightning againts the biggest 2 armies in the world (soviet union, germany) is told to be "weak in tactics and experience" while still holding on for a month.
Poland had more troops than the British Empire during 1939, so where do you get "smaller Poland" from? Doesn't change the fact that the British Empire defeated Germany while Poland lost to them.
Fightning on all fronts? What fronts? They were fightning on one front. Africa, againts italians.
The British were fighting on all fronts from an economical and military perspective. They were busy fighting the Germans militarily in Eastern Europe, while financially supporting and militarily supporting the Mediterranean countries; let's not forget about Japanese aggression and movements in the Pacific and SEA, AND the Middle East on top of all that.