I'll be studying the MSc Computer Science course at Birmingham so I can give you my opinion for what it's worth. As others have said, I think it largely depends on your career plans. If you're thinking about Software Engineering/Development then the course is a really good way to get your foot in to the door for companies that require a computer science degree, however many large companies don't have this stipulation. If you're more interested in the UX modules then I'd also recommend MSc Human-Computer Interaction (HCI), which UCL also offers.
I'm sorry to hear you struggled during your undergraduate studies, please don't let this put you off considering further study! If you have a diagnosed condition then you can go to the Universities disability support service. They can offer mentoring services and will provide you with relevant support throughout your studies (such as extra time in exams, extensions for assignments etc.). Since the conversion courses can be quite intensive, its even more important to try maintain some sort of healthy study-life balance.