As a fellow Brit who has been using Sketchup on and off for over 15 years, I agree with you that there is a lack of truly joined up tutorials. That may in part be because different people use SU for completely different things. You and I work in the architectural field but others want to design car and other curved surface objects.
As you say, there are many good (and quite a few not so good) online tutorials. I found them very helpful when I started but I haven’t come across many for the advanced user, apart from ones to do very specific, and sometimes rather abstruse, things.
I have only been using SU as my main CAD software for about 5 years. I found Michael Brightman’s Sketchup Workflow for Architecture enormously helpful when I transitioned from AutoCad. Nick Sonder has produced a book that looks to be in similar vein and is on my wishlist even though I guess I am now quite an experienced user. I still learn new tips and tricks through this fabulous forum where you will find a host of really helpful people, of whom @DaveR who has already responded here, is one of the best. He is also almost always the first out of the blocks when it comes to answering people’s problems. It’s almost like having your own personal CAD guru inside your computer!