Moan away that’s what forums are for isn’t it? And in your case you’re definitely entitled to as that sounds horrendous! That lack of compassion and empathy is unprofessional and I’d be using the complaints system to flag it.
On a more practical side maybe your practice nurse could assist with dressings and advice? Most of us don’t have medical training and you should at least be given dressings rather than having to buy them yourself. Having said that I did buy some showerproof dressings from Boots preop and their waterproofing should work both ways so they might be useful to avoid a recurrence of the shop incident. https://www.boots.com/elastoplast-waterproof-sterile-dressing-xxl-5s-10321421
There’s some post op info here After breast cancer surgery | Breast Cancer Now Also this may be reassuring from the Royal Marsden
https://patientinfolibrary.royalmarsden.nhs.uk/document/download/1459
I hope it resolves itself soon x