Hmmm. Trouble is, they've allowed you to go on full sick pay because they would prefer it, you said yourself you weren't actually not fit for work, it was a travelling thing.
Have checked Equality Human Rights for definition of pg-related sickness
'It is not always clear whether an illness is pregnancy related. Some conditions are partly related to pregnancy and sometimes pregnancy makes existing conditions, such as back problems, worse. In either case if a woman has to take time off work or is otherwise affected by her pregnancy it will be pregnancy related. '
So if your diabetes were affected by your pregnancy to the extent that you are having to take time off work, then it's pregnancy-related. But your issue is you had a crash caused by your diabetes and now can't drive as a result. So the question would be did your pregnancy exacerbate your diabetes and make it more likely to happen. I'd be asking for a medical opinion if I were the employer. Have they done that, and why are they assuming the incident was caused by your pregnancy?
Problem is, as you said, it's only the driving that's a problem, you didn't actually need to be signed off sick, but rather than faff around with cabs or homeworking, your employer has said you can go off on full pay then half pay. So insisting that it isn't pregnancy related and saying you'd rather stay off sick might well backfire on you, they may say in that case work from home or get a lift in and come to work iyswim.
It's iffy whether it's better to be pg-related or not. There is the reference/disciplinary issue where pg-related sickness can't 'count', but obviously it does mean they can start your leave when it gets to 36 weeks,
Tbh I think as they have allowed you to go off with generous sick pay which they needn't have done, you might be best off giving in and starting mat leave at 36 weeks, to avoid loss of goodwill on their part. If you want to stand firm and insist it is not pregnancy related, I would be prepared to provide a medical opinion along those lines, from your GP or specialist.
HTH, and hope you are feeling ok