So I recently heard that Britain would have joined WWI wheThere or not the Germans had attacked Belgium. Is that true?
Yes and no. The answer is complex, but the short of it was that Britain didn't want German victory on the continent and Belgium was a convenient excuse to enter the war. With an OTL situation (declarations of war being tossed between the Central Powers and France-Russia), Britain's entry would've been delayed for a time until a proper excuse pops up and Britain fully enters the war.
If the war doesn't break out over Sarajevo ... then things get a bit more iffy. As much as Britain and Germany were at odds, British relations with Russia were deteriorating. If the war is delayed long enough, the relations might cool off enough that Britain winds up clashing with Russia itself (remember, as much as it was a paper tiger in retrospect, Russia
didn't appear such to contemporaries, to the point of Germany believing a war with Russia unwinnable after 1917).
In such a case, Britain would, at best, be pro-French neutral and try to preserve French independence, but might not be willing to choose between two equally bad options (i.e. Germany and Russia). But that's not a certainty and Germany is quite the threat, being in Britain's immediate neighbourhood, unlike Russia which, at best, threatens India.