I've been using the Ribbon format for about 3 years now and I STILL hate it. The newer versions of Outlook are the worst - the combination of the ribbon and the way MS couldn't be bothered to reimplement the compact header format really eat up vertical screen space for those of us who prefer the bottom preview pane layout (yes I know I can hide the ribbon but then I lose all the buttons which do what I use all the time, which is mainly the quick search box). On a laptop with only 768 vertical pixels (when I'm not docked) that is a serious headache which leaves me using OWA instead of the full blown outlook usually.
As you point out, the 2003->2007+ switch was therefore a huge opportunity for OpenOffice/LibreOffice/whichever fork is your favourite. The UI is great, easy to understand and the small differences from Office 2003 (like where the cursor ends up mid-editing a formula in Excel) are actually mostly positive incremental steps. You theoretically get the usability benefits of 2007+ (particularly for Excel, where memory/size constraints in 2003 were getting to be a problem for many).
Unfortunately though, interoperability is extremely poor - LibreOffice simply can't handle a big Excel spreadsheet (which is in my experience at least 60% of what most businesses buy Office for), and I've sent docx files from LibreOffice where, when people open them in MS Word, all the line breaks are suddenly gone or other formatting oddities appear. As another example, trying to use LibreOffice's "track changes" equivalent functionality left me with a docx file that Word (and often LibreOffice itself) is unable to open.
I would love to think that if the UK Government does move to LibreOffice they would fund someone to provide decent support who can fix a lot of these issues - that is supposed to be how the model works and fixing these issues would be of huge benefit to everyone. Unfortunately I can't really see that happening. I suspect instead it will end up being a typical government cock-up and massively overspend/under-deliver. I just hope that people don't end up viewing "Open Source" as the problem reason as it will be nothing to do with that and entirely to do with yet another display of civil service incompetence.