Cranberries are not needed for a UTI; I have never found that it has made any difference with piggies of mine, to be honest. It also doesn't affect the natural alkalinity of guinea pig urine, which makes guinea pigs more prone to urinary tract problems than other species.
Extra vitamin C will do to help strengthen the immune system. Even more useful is feeding mildly diuretic veg like young dandelions, lettuce, cucumber or celery chunks to help flush the bladder and ease the acute symptoms in the early days. (Don't overdo it, though, so you don't end up with squishy poos or diarrhea!)
Cranberry juice may help in some cases when you are dealing with an interstitial cystitis (a recurring inflammation of the bladder walls) according to some people, but again, it has not made any difference with any piggies of mine.
Did you get some metacam (anti-inflammatory and painkiller) together with the septrin or just septrin?