Hi!
The licensed UK brand of septrin for small exotics pets (which includes guinea pigs) is called sulfatrim. It comes in fluid form. Your vet should be able to order it for you.
Please be aware that you may be dealing with a non-bacterial recurring bladder infection (i.e. a sterile interstitial cystitis or sterile IC) that cannot be healed by antibiotics. It is an issue that has become increasingly common over the last decade but that is not much known outside vet circles that see guinea pigs on a regular basis.
Management of IC is mainly with glucosamine in order to support the natural glucosamine lining of of the urinary tract which prevents corrosive urine from coming into direct contact with raw tissue. This coating seems to be rather affected by sterile IC. We recommend to give 1 capsule of Feliway cystease for cats daily (the other species with a sterile cystitis problem, called FSC) by mixing the contents of the capsule with 2 ml of water and then syringing either all once a day or 1 ml twice a day; this is the easiest way re. dosage and convenience of giving as you don't have to grind down any tablets, having to compute how many doses you get out of them and mixing them with the appropriate amount of water.
For severe cases of sterile IC, cartrofen injections are currently coming into use on an individual basis after recent research has shown that they are also effective for guinea pigs with sterile cystitis; but this is still in the early stages and there is not yet a consensus of what is the best dosage and interval. Cartrofen is a high glucamine based arthritis medication.
PS: Glucosamine is classed as a food supplement in the same category as vitamins or probiotics and not as a prescription-only medication (POM) but cartrofen is prescription-only and not officially licensed for guinea pigs. You want to first trial the capsules and see whether they have a noticable impact over the course of several weeks as they need a bit of time to build up. I
The other medication used is metacam/meloxicam (which comes under a range of different brand names these days); it is an analgesic (i.e. painkiller cum anti-inflammatory). You adjust the maintenance dosage to a higher one during acute flares.
Red porphyrine coloured pees, which may or may not test for blood, are characteristic for the onset of an acute flare, which usually happens every few weeks. Sterile IC can usually only be diagnosed by eliminating all the other possible urinary tract problems (which in your case has now happened). It cannot be healed, only management until it hopefully goes away on its own eventually - more likely in a matter of years rather than months. The good news is that it is generally not life-shortening and that is also doesn't cause any bladder stones.
The big teddy in my avatar, Nerys, had sterile IC for 3 years and then another 3 years free of any bladder problems until her death, aged 8 years, if you find that helpful. My current IC piggy is Breila.
IC can be transmitted via sitting in a pee puddle or via scent marking (the tiny scent gland is located just in front of the genitalia) but healthy piggies will fully working immune system can usually fend it off without ever showing acute symptoms. If you have more than a pair of piggies, please make sure that you wash any bedding and cosies separately and that you disinfect your hand and any other surface in contact with an IC before you put other piggies on it; including the lawn unless it has been raining in the meantime.
Like with cats, sterile IC seems to typically affect guinea pigs with high stress levels as their 'normal' default setting; since this happens while still in the womb of a highly stressed mother, there is nothing you can do to influence this unfortunately. The current indiscriminate mass breeding of all pet species whether via commercial pet shop suppliers or for-sale breeders sadly does have consequences. Sterile IC in cats and guinea pigs is unfortunately one these.
Some more information on IC via this link:
Links - Interstitial Cystitis - Guinea Lynx Records
I hope that this helps you?