There's an irony there, in that looking at Simon K's blog, Britt was very fussy about what Thomas etc should look like - yet in some instances, like the dimensions of Thomas, it was the TV company who'd got it wrong!
Though I agree that the illustrations in the book aren't always terribly accurate either - both Henry and Thomas ended up having accidents and going away to the Works to be 'rebuilt' to correct errors in Dalby's drawings (Henry also wound up with Gordon's buffers in one picture - when a child wrote in to RWVA to point this out, rather than replying that "the artist made a mistake", RWVA wrote the child a story about how Henry had damaged his buffers and the Shed only had spares of Gordon's so Henry had to use those until his spares arrived!).
The line in Branch Line Engines where Thomas calls Percy "a green caterpillar with red stripes" was originally used by RWVA to describe Dalby's drawings of Percy, which led to Dalby resigning from the series.