You're not catching what I'm thowing, but my PC is broken so I'm on the PS4 which limits my responses.
I am aware that if Germany wanted an alliance with Britain it would have to make concessions, however given the value of such an alliance would those concessions be too great? Germany wanted a big navy, they could afford to pay for a big navy and during it's construction the Army was artificially kept small to keep middle class officers out. It is no coincidnece that the year the navy abandoned the so called naval race was the same year the Army expanded by some 135,000 men, the resources allocated for defence went to Army expansion rather than navy expansion. Why should Germany lose this big navy in order to enter into an alliance with Britain?
You keep saying Britain didn't choose a side, well from 1902 after the alliance with Japan the RN started redeploying squadrons back to Europe, so the threat had been identified. By 1906 Britain had chosen a side, the Germans didn't call the British King 'Eddie the Encircler' for nothing. By the start of WW1 Britain only had a single plan, deploy the Army on the French right flank, this is well and truly chooing a side.
As for spuer power, most of the countries that were great powers in the 1800s hadn't become weaker, but the goalposts had moved and it was possible to accumulate much greater power from vast territories. Frane and Britain were limited in their expansion however Germany was already much bigger than both and had on its borders subject countries and peoples who could just as easily add to the wealth and power of Germany than to their current overlords Russia and AH if Germany could get hold of them. WHat's more this is a much more secure way of gaining resources than overseas colonies which as both wars showed were hideously vulnerable to uboats.
Well to court the UK, Germany would effectively have to convince the UK that British interests would be best served by supporting Germany in a conflict, so lets see what early 20th century Britain wants: First of all the desperately need to keep naval supremacy, as the disadvantage to being a heavily populated island is that you are incredibly vulnerable to blockade (indeed in the 1920's British defense planners estimated that if a larger enemy fleet arrived offshore the UK would be forced to surrender in a matter of days). That's one bargaining chip the Germans could have; offering the deal they did in OTL much earlier, before they had effectively lost the anglo-german naval race and the issue had essentially already been played out(because unlike the UK they need to put a lot of resources into building up a big army, as you said). The other issue is the British don't want one absolute power dominating the continent. Germany is already the strongest continental power so to get British support it would need to lay out exactly what territorial expansions it would demand in a victorious war with France and Russia and these gains would likely have to be limited. Some politicians like Bismark understood this and managed the British quite well during international crisis in Europe (the Franco-Prussian war being a great example) as well as diplomatically isolating France. If the Kaiser tries to immediate Napoleon and just grab huge swaths of Europe (arguably OTL) then the UK will move against him because one country dominating all of Europe is an intolerable existential threat to the UK.
Also I am not saying the UK never chose a side, eventually it did as it sided with France and Russia, however it was quite late to joining such an alliance and even when there it did not immediately come to the defense of either county at the outset of WW1 (using the violation of Belgian neutrality as an excuse). My point here is that unlike France, and to a degree Russia, Great Britain is not irredeemably set as an enemy of Germany. Unlike most other Great Powers in Europe (with the notable exception of Italy) Britain isn't automatically in one camp or another and when war breaks out isn't directly and immediately threatened. By 1914, it is true that the UK had firmly fallen into France's camp, but it didn't have to be this way, if Germany had taken a different diplomatic approach to courting Britain in the late 19th and early 20th century. There were certainly fault lines between the UK and France that could have been exploited by Germany (eg. the Fashoda incident).
You are right that France and Britain of the early 20th century aren't head and shoulders above everyone else like they were in the early 18th century. A now fully unified Germany has certainly caught and surpassed both in terms of home population and industrial capacity (especially France), however you are incorrect about expansion. The UK expanded by settling overseas (in this age Australia and Canada were the popular destinations) and there were huge waves of emigrants heading out to the empire and the "White Dominions" in particular. France's colonization was more limited (and it also had a much smaller population) but ethnic French people were also settling North Africa. Germany was a very densely populated country (ie. Germany as "a people without space") and had few colonies and essentially could only expand by warring against its neighbors on the continent to acquire more territory, the less populated territory to the east was especially enticing (as it would remain so into the 1940s ie. the "lebensraum"). Indeed one of the big targets of German emigration at this time was the US, which today actually has more people who are ethnically have German heritage than English heritage. The dense population and heavy industrialization of Germany meant it also had to import significant amounts of resources and materials to keep its economy running and to feed itself, and in war times, if the sea-lanes were closed Germany would be forced to seize resources from its neighbors in order to make up the gap (ie. this is why the blockade of Germany was very effective in the later years of WW1). As to the submarine threat: in WW1 the treat is less, due to Germany not having the bases in Northern France to operate its subs out of that they would have in WW2, greatly limiting Germany's ability to raid convoys to the UK. It is true that Britain is immensely vulnerable to blockade (it is much more reliant on imports than Germany or any other country for that matter), but even in WW2 the U-boats were never able to cut transatlantic traffic to significant levels, although there were times when Germany was sinking merchant ships faster than they could be rebuilt and the threat was definitely looming. However it is famous knowledge that in the UK during WW2 bread was never rationed until after the war, when it was being sent to feed West-Germany (and to be fair the wheat crop in the UK in 1946 failed).