It happens all the time and will only get more common. I would say you are quite unusual in knowing a TA will be teaching, many parents have no idea their child is being taught by an unqualified teacher.
Can anyone teach if someone else plans the lesson?
I would argue that they can follow the plan, but 'teach', who knows? I hate following other teachers' plans and usually adapt the plan according to the assessment as the lesson proceeds, in fact I do that with my own lessons too.
If a teacher is planning the lessons for the TA then the teacher is responsible for assessment.
I think it is outrageous. Many TAs are great, so they should qualify and earn the correct salary for teachers. I know many TAs who are planning, marking, assessing, doing parents' evenings etc and are essentially a teacher at a bargain price.
IMO ALL primary teachers should be qualified as they are setting the foundations. Of course independent schools have always been able to employ anyone and now academies can too. Something to thank Gove for.