Hi there, I'm a new OU student just starting the Maths & Stats degree- we seem to have similar interests as I'm also planning on doing some computing/ICT along the way and am currently studying a science module (S283) as my very first OU module. I'll be starting MST121 in February as well, which means we'll probably be taking MS221 & M208 together as well!
My comments on your degree plan would be:
* If you have O level maths (even if it was a long time ago!) I would say you probably don't need MU123- if you're comfortable solving basic algebraic equations, I think you'll be fine (anything you've forgotten about trigonometry, factorisation etc can be revised prior to the course when they send out the revision pack!) But obviously there's lots of material on the OU website to help you make your choice. From what I can tell from MST121 materials that I've seen, basic differentiation & integration is about as hard as it gets on MST121 (not to say that's simple, I only have GCSE level maths myself so I'm a bit nervous!)
* The Computing department is messing around with its degrees at the moment so you should be aware that the last presentation for M366 is in February 2013 (which at the moment I am planning on taking!). The last presentation for T224 (Computers & Processors) is currently Feb 2012. It's not looking like there will be direct replacements so if you're really keen on either of those courses I would get them in during last presentation.
* M381 (Number theory & mathematical logic) & M336 (Groups and geometry) are having their last presentations in 2013 and will be replaced by a new 60 pt L3- M303 Further Pure Mathematics (which covers number theory, groups, some topology, rings & fields- no logic!)
* M373 (Optimisation) is currently not scheduled for presentation in 2014 according to the Maths department's presentation document (looks like it's scheduled for every odd year from 2013), but if you're interested in pure maths M337 (Complex analysis) is being presented in 2014.
The OU Maths & Stats pathway advice forum is super helpful when it comes to planning any degree that includes a lot of maths- if you're registered for an OU course you can access it here:
http://learn.open.ac.uk/site/maths-stats (requires login to studenthome, then the forum link is on left hand side of the page.)
Hope this is helpful!