Generally, the ones you can get on the NHS are:
- somaErect
- Farnworth
- Osbon Erecaid Esteem
If you google them on pharmacy pages, the NHS number is usually displayed in the item description - see here for example http://www.mediplus.co.uk/c-Osbon-ErecAid
John has the Osbon Esteem but we also used to get the Farnworth rings as they were a tighter fit and slightly easier to remove than the Osbon ones.