I'm sceptical TBH Wilder of any med in the UK claiming to cure internal parasites that isn't medicated food or a paste to medicate food with. I have not yet found any active ingredients that definitely cure IP's, and that are legal in the UK. The only thing that's legal and close to getting IP's is Flubendazole-5, used in Wormer Plus, but a dose strong enough to get all IP's with that type of Flubendazole will kill the fish you are treating also...
If mattlee, you want to treat for internal bacteira and/or internal parasites, you want to apply a dose of Interpet Number 9, and a double dose of Myxazin, at the same time. It's a very much "kill or cure" treatment process, that usually kills as many fish as are already infected with the internal bacteria. The fish with internal bacteria will be killed by this treatment also... However, us aquarists have been breeding IB infections that are medication resistant, and hence I find Interpet Number 9 useless on it's own nowadays...
For internal parasites, get yourself some Beaphar Wormer Plus reptile wormer and some frozen food. Weigh your fish (or if you are feeling risky, guess their weight), and add as much paste to a cup as you would for a reptile of the same weight as your fish. Add the frozen food to the cup and allow it to stew for about an hour, before draining and rincing the food in a fine net and feeding. Feed only medicated food and re-apply the medicated food daily for a week.
Note, both those treatments are not being applied in the way they were intended to be applied. Mixing medications is risky, and using medications for applications they were not intended to be used for is likewise risky. Use the above guidance at your own risk. However, I know of anything off the shelf that actually works on either or both these problems with fish. I however have used these methods before in the past. I haven't treated for internal bacteria and internal parasites at the same time however, and mixing the treatment may get you problematic results...
All the best
Rabbut