I don't know how much help my experiences will be, because although I did my OU undergraduate degree 'for me' that actually led to me choosing to do the Open Degree rather than a named programme. Basically, I knew I wanted to do the Creative Writing modules, so initially I looked at which named programmes those were available in, so I was thinking about English/humanities type degrees. At level 3 there was another module that interested me a lot within those routes; and at level 2 there were some things which looked fairly enjoyable. But, in order to get to that point, I would have had to do AA100, and the more I thought about it, the more I didn't want to do it. And that would have been a year of study, and although I know lots of people have really enjoyed that module, it didn't really speak to me. At all.
Whereas, when I looked at the wider selection of level 1 modules, there were other things which sounded much more enjoyable (especially U101* aka 'Lola'). So, I thought sod it, and did the Open Degree, so I could just do modules I actually wanted to do. It meant I could combine the Creative Writing modules with the Design and Innovation ones - U101, T217, T317 - and effectively build a joint honours degree. The two subjects complemented one another really well - my A215 EMA, for example, was actually inspired by something I'd read about in U101.
So, I guess my advice would be to look ahead as far as possible, and see what modules would be compulsory in each of the degrees you're interested in, and what would be optional, and whether they sound good to you. I also looked at the pass statistics for the modules I was interested in - obviously they can vary a bit from year to year, but where a module had a pattern of lots of people dropping out/failing/getting lower grades, I tended to be a bit cautious as to why that was.
In terms of credit transfer, you'd need to dig out the relevant details and see what level they were counted at, and whether you completed whole modules (transferrable) or not (not). You'd also probably be limited in what you could transfer in to a named programme (i.e. if you chose, say, an English degree then credits related to English would be more likely to be accepted, although there's sometimes scope to bring in a teeny bit of something unrelated).
(*U101 was, indeed, ridiculously fun.)