Have been through all the legalities and acceptable uses of TAs in the classrooms over the last 3 years in quite some detail with school, the LEA, and the unions, plus the official paperwork sent from more central.
A level 3 TA can cover a class short term if unplanned - ie a non planned teacher absence. This can be for up to 3 days apparently, many schools will be for first day sickness only. This should be under the supervision of another qualified teacher - doesn't have to be in the classroom at the time though.
A level 4 HLTA can cover lessons, and is generally expected to plan, teach and assess. This can be for longer term such as PPA cover, but shuld not be in replacement of a teacher long term - ie not covering long term sick or maternity pay.
Some TAs are qualified teachers - more common than people may realise infact. It is possible for one of these TAs to be given a temporary change to their job - and pay - to work as a teacher, like a supply teacher would. This does happen and again is acceptable.
Some schools, however, do try to use TAs as supply teachers - this is not acceptable and actually isn't fair on the children, the TA or the teacher either.