There was the "Invasion Scare" of 1859-60 when elements of the British public seriously feared a war with France. Tennyson wrote "Form Riflemen Form," and volunteer rifle companies were established across the country.
This brings up an important point: in the event of war with France, Britain always had to prepare for the threat of French invasion, as well as the possibility of a French landing in Ireland. The Channel isn't that wide - at its narrower points, you can see the other side on a clear day. Crossing the Channel with an invading army is of course quite difficult, as history has shown. But it was not out of the realm of possibility. If Britain could avoid this possibility by not going to war with France, that would be in its best interests.
Conversely, Germany (if it hasn't occupied any of the countries to the west) would have a logistically more difficult time striking directly at Britain, and Russia would have almost no chance of doing so.