How long have you been a SAHM for? What was your medical qualification?
Having other professional experience does sometimes help in getting pupillage, but depends on what you did and how recent was your last period of work.
As another poster has said, you may find an OU degree doesn't carry as much weight as does a good red brick uni. Unless you are likely to get a first, you may be better off just doing the GDL.
You would be unlikely to get a pupillage in a commercial or chancery set with an OU degree, but may have more luck in a criminal or general civil law set.
You would probably also need to get some relevant work experience- volunteering in a law centre is a good idea. Doing 5 or 6 mini pupillages is a good idea, and will help you select the right sort of set for you, but it is the minimum that any applicant will do so won't help you stand out.
There is also much talk of bringing in alternative business structures so that barristers and solicitors work more closely together. The structure of our legal system is likely to look very different by 2017. The solicitors' qualification is likely to be the most versatile of the two, unless you already have an established reputation.
The truth is that only around 25% of people who complete the bar course ever get a pupillage, and this is likely to reduce in the next few years if cuts to legal aid go ahead. You should be prepared for the fact that it is statistically unlikely that you will secure pupillage before starting the bar course, so should have a plan for how you will gain relevant professional legal experience whilst you keep applying.
My advice is that it is only worth going to the bar if that really is the only job you can see yourself happy doing. If there is anything else at all you would be happy doing- do that instead.