My manager says we are getting jumpers that we have to wear while the shops is open. Can they make me even if I'm hot?
Generally speaking (because we don't give legal advice and you did not mention your jurisdiction) yes, your company can make your wear their dress. I know countries where that means they have to pay for your dress, but I don't know any country where you would get away with not wearing the dress your company wants you to wear.
If it's very hot and the jumper is hindering you in doing your job most efficiently, talk to the other employees how they cope and if most of them have this problem, talk to your manager and suggest an alternative (wear company shirts instead maybe).
If it's so hot it's threatening your health, talk to your doctor and then to your manager. That might be a valid reason to have a different kind of company dress.