Hi Angel,
I'm in the UK and yes, you must obtain the Diltiazem through a GP prescription.
I am one of the "success stories" using this - I started on the GTN, but it gave me really bad head rushes, so asked to switch, which was no issue. I used the cream religiously for about 6 weeks or so, twice a day, and noticed an almost immediate reduction in pain relief. I was initially afraid of coming off it but it had worked well enough to allow my fissure to heal to such an extent that it was no longer creating all that horrible pain and it went on to fully heal in the end.
A stool softener is also key (I used Lactulose) and hot baths/ hot water bottle also help with the pain relief. It sounds like your mum is a great support and doing all the right things to help you, which is brilliant!
My tip when you see your GP is to be completely honest about your pain levels and if needs be,
exaggerate even to get your referral to a CRS as soon as possible. If the worst comes to the worst and this is taking time, you could consider setting up your first consultation as a private patient (which OK, is costly) but it gets you into the "system" - this is what I did. I started as a private patient and once diagnosed, requested to continue the treatment under the NHS.
I wish you all the very best with this and hope you are able to find some relief very soon :)