"The UK should not use its armed forces to break International Maritime Law, period."
True!

But the RNLI should not be sending boats to meet boats which have been escorted by the French authorities to mid-channel.

Not meeting them does not put them in peril. The smugglers did that by arranging the boats.
The French police were complicit in allowing them to leave French shores.
Any boats escorting them to the mid-point are also complicit.

Why does it only seem to be the British who say "Well, although they put themselves in harm's way, it is our responsibility to rescue them."?