OK, this is my usual response when someone mentions taking a ferry between England and France by way of the Channel Islands:
I realize that I have been uncommonly unlucky, but I have booked five such ferries (all on the St. Malo route), and two of them were canceled. All trips fell between May and August, so we're not dealing with violent winter weather here. The most recent occurrence was in 2017, and the ferry didn't run for three days. Something like that could cause serious problems for folks on a tightly scheduled trip. EasyJet has flights between London-Luton and St. Helier, Jersey. Probably there are other options for getting to or from the islands. Definitely do some research so you know what your back-up plan is, and monitor your email so you'll learn of any cancellation right away.