If your vet is a knowledgeable rabbit vet, they will often carry the probiotic. Your local pet shop may have it, or you can buy it online, which these are usually the cheaper options. They have instructions on the container for what the typical administration will be. Usually it's during the course of antibiotics, though some rabbits with digestive issues, they seem to do better with having it daily. Just depends on the rabbit, what treatment is occurring, and what your knowledgeable rabbit vet advises.
Some other probiotic options in the UK are, Vetark pro c professional, which is probably more cost effective as you get more for your money. But it does contain added vitamins, so shouldn't be given long term if vitamin enriched pellets are also fed regularly. Same with Bio lapis. Fibreplex is just the probiotic, prebiotic, and carrier oil and thickener, so more suitable for long term administration in my opinion. And there's Protexin pro fibre, that is a probiotic pellet, and that is meant for daily administration according to the manufacturer. Though it does contain added grains, so some rabbits may be sensitive to the added carbs to the diet.
If this is something you do want to give daily ongoing, if you find it is seeming to be beneficial to your rabbit and your rabbit isn't sensitive to the added grains causing mushy cecals, then I would say the pro fibre pellets would be the best thing and easiest to administer(if your rabbit likes eating them). Though do consult with your vet, if not before you start them, then when you get your rabbit in for the appointment.
I've only given probiotics during antibiotic treatment(making sure to give a few hours after antibiotics were given so the antibiotic doesn't affect the effectiveness of the probiotic), though I have read of it helping some rabbits with their ongoing digestive issues when they were given daily. So it just depends what you and your vet determine works best for your rabbit.
One thing I will mention, is if your vet isn't particularly knowledgeable about rabbits, and especially if you have any concerns your rabbit is getting the proper diagnosis and treatment, then finding a more knowledgeable rabbit vet is usually recommended.
https://rabbitwelfare.co.uk/rabbit-...abbit-friendly-vets/rabbit-friendly-vet-list/