H&M have some puffer style jackets that might work? You can also get these individual inserts https://www.zipusin.co.uk/
Dry robes are good for this tbh
But there's loads of cheaper versions too
Seraphine do jackets with an insert that goes round a baby carrier so you can have a regular jacket or one that goes around the baby carrier/bump
Try Vinted too. Some seraphine ones on there but have managed to get an equivalent one from George at Asda through Vinted which seems good and has the removable panel too
Try keeping an eye out for mamalila coats in vinted. You can use them for baby wearing front and back. Not cheap but they come highly recommended
I plan on wearing my dryrobe. If you get one from Regatta or something rather than an official Dryrobe they’re much cheaper.
I’ve got a JoJo Maman Bebe coat which I bought from Next. It has a removable panel so you can wear it postpartum too. I’ve also got the carrier panel too ready for baby wearing. It’s lightly padded and showerproof. For me, it was a cheaper option than Seraphine (and they barely had any sizes in stock when I looked). I’m a bit annoyed by the lack of pockets and no fastenable pockets though!
I'm a professional dog walker, I've got Agri waterproof coats from the Agri stores and they are the best!! Very long lasting, maybe not the most fashionable.