London University External is essentialy the distance arm of the University of London Colleges e.g. Kings, Goldsmiths, Birkbeck etc - all these colleges have good academic reputations. I dont have first hand experience, but for me it's all about quality of materials and support.
As for the OU, your academic progress is through feedback from assignments. A tutor will give you reasonably detailed feedback on your work and how you could improve the next time. It'a actually part of the tutors role to do this, if they dont you can go up the line and complain. On most courses it's a 50:50 split with continuous assessment and exam in terms of the final grade for a module. It'a all quite transparent, you get a study guide and assignment guide and everything is clear re what they expect from you to achieve a good grade.
From my experience it's important not to overload yourself with too many modules at once. You may want to get a qualification quickly, but a full-time degree takes 3 years, so if you are starting from scartch with the OU i.e. no credit transfer, getting a degree in 4 years will be heavy going.