Mebendazole is sold in the UK without a prescription. The standard dose is for pinworms.
The other drugs I took for Dientamoeba Fragilis - Iodoquinol and Doxycycline I ordered from USA. I got the tests done myself using a private clinic in the Uk and had the results sent to my vet who forwarded them to me. I took the results to a doctor who I knew had some knowledge of parasites and just used him to write a prescription to send to USA. If you are thinking of using the drugs do plenty of research first. Get tested. Some drugs are appropriate for some parasites and not for others.