The degree at the Open University, as you've rightly said, is for people already in healthcare roles. This is because your employer would work in partnership with the OU and provide your placements, unlike other universities who organise your placements for you.
If you're not already in a healthcare role, doing your Nursing degree through the OU is quite a long-winded, rather uncertain route honestly? you would need to A) secure a role as a healthcare assistant or other caring role where practice supervision & assessment is available and in line with the NMC's guidance and then B) work in that role for a while, build trust and rapport with staff/your employer and then see if your employer will agree to support you through university (which of course is down to their discretion), before you then apply to the degree.
Is there any particular reason you want to go with the OU? other universities don't require you to be already employed in a healthcare role beforehand, so that would eliminate the added steps/time that it'd take to study with the OU?