To expand on that,
You either need antibiotics or you don’t. If you do a GP will prescribe them, if you don’t then a) taking them won’t make any difference to whatever condition you’re taking them for and b) by taking unnecessary antibiotics you’re increasing the risk of antibiotic-resistant bacteria evolving. And when that happens we’re all screwed.
It’s not even a cost saving. A prescription is, what, 8 quid? A quick google would suggest that online antibiotics will cost you three times that (though it also comes with a free pack of Viagra).