I think the esha2000 is probably the best bet you can get over the counter in the UK. But i just want you to be aware that it isnt an antibiotic treatment. Nothing in the UK legally obtained without going through a vet is antibiotic. It might have antibacterial properties, and may help, but its a shot in the dark. Think of it as trying to treat organ failure with a herbal remedy. Some people will pull through, most wont.
Dropsy is a symptom rather than a cause, and many things can cause dropsy. Bacterial infection, parasites and organ failure are common causes. You need to treat the cause and its very difficult to determine the cause. Also, by the time your illness shows the dropsy symptoms your fish is often beyond saving. Esha2000 will treat a wide range of issues, salt also. Keeping water pristine will be your best route to recovery.
Hopefully your treatment works ok and your fish recovers.