You can by neosporin and bacitracin in pharmacies here in the states...they are antibiotic ointments. I have used Hibiclens before...which is an antibacterial cleanser that is also sold in pharmacies...you put a couple of drops of it into a cup of water and then dip a cotton swab in it and dab it gently on the infected area a couple of times a day. I'm not sure if they sell that in England though...but we have a couple of British members on here that might be able to recommend something if you can't get that. More pics will help...and we can look at the rest of her to make sure everything else looks good. If you can show us a pic of her underside...so that we can see the lower tail area, we can confirm if she is female too.

Welcome to the forum and to owning a garter snake!