True, but it's still hopping through a neutral country.
One first has to assume that British leaders and public care about neutrality as a matter of principle. In fact, I think they don't. When Britain decides it's in her best interests to remain neutral, then they care about the principle deeply. When other countries resolve to remain neutral in a war that Britain supports, then neutrality is a contemptible evasion.
See how Britain treated neutral shipping rights in WW1, often violating the very same principles she insisted upon when she was a neutral in the ACW and Russo-Japanese War.
Britain's respect for neutrality as a concept was entirely subject to her interests at the time.
And keeping Belgium out of the war is an enduring feature of British interests.
Edit: Well, partly. When Churchill (a hawk if ever there was one) looked at a map of Belgium, he exclaimed that there would be no pretext for war if Germany only invaded the southern bit. Other Cabinet members apparently agreed. This suggests that Britain's commitment to even Belgian neutrality was not truly a matter of deeply-held principle, but rather of policy: an invasion south of the Meuse-Sambre didn't threaten the ports that Britain was concerned about.
As Germany (no longer France) was the nation encroaching on Britain's position of preeminence in trade and industry, and as some British leaders considered it necessary to appease the Franco-Russian Entente, France will never attract as much official hostility over invading Belgium, as Germany would in the same circumstances. And as noted, an invasion of southern Belgium is not as threatening as an invasion of central/northern Belgium. However, an invasion of even southern Belgium, even by France, must attract some onus. If Britain is already on the fence, it could be very significant.