The Rock Channel is a useful half tide access to Liverpool for small boats (which includes large yachts) as opposed to ships. This route can cut 5 miles off the journey and can also be a lot calmer in my experience than the swell you can experience at the entrance to the main channel.
I’ve put the following two links in which you can choose between. The first link is older but has some nice chartlets and a historical article about the Rock channel. I followed this route with success on the way back from the tall ships race and never had less than 4m under the boat 2 hours before high tide. The depths were similar to our standard trip in and out of Conwy.
Liverpool Yacht Club Guide to the Rock Channel can be accessed from this link. Updated 2010.
Alternative guide at Chris Michael’s page on liv.ac.uk can be accessed from this link. Updated 2013.