Make sure to ask your GP for an iron infusion, as if your levels are as low as they sound you'll be eligible. It might be good to have one or two then use oral iron pills as well, whilist the mirena kicks into gear.
Also as the mirena settles, ask for tranexamic acid or northisteron from your GP - a smallish dose, to use occasionally when your bleeding is still heavy as the mirena settles.
They may not have your previous failed attempt on record. Obviously they won't do general just for a coil, they'll be doing another op and then might as well do it at the same time sort of thing. However if you do want the complex coil clinic you would be waiting longer as it is the gynaecology ward, and so a F1/F2, some more trained nurses are needed, and they'll be using them for ultrasounds, and other things, so they often don't do many clinics across the month, so it can be a few months wait often. I'd still go on Tuesday, make sure they have everything and then hopefully it all goes fine.
Have you called them yet?
I hope it goes well for you, the pain relief should definitely help. If you have Co-codamol take 1 or maybe half of 1 (if it is the over the counter dose, maybe 1 as it is a lower dose). It is capped at 8/50mg, whereas prescription ones can be higher. Obviously don't use it regularly nit good if something worse happens and addiction risk etc. You could also use the paracetamol and ibuprofen definitely 1h before, and after if it very painful use 1 Co-codamol, but get the hand warmers I also mentioned ready, as they help.
Do you have pain with your bleeding? Of so ask your GP to prescribe you an NSAID so naproxen, or mefenamic acid or other ones. They help with pain, but crucially always take it with food and omeprazole, and also start taking it before the pain starts, so a few days before your period should start. For tranexamic acid or northisterone, initially for your next period and maybe the 2nd one as well, use it before your period should start as well, but then as the mirena settles, only use it occasionally in a day where bleeding is getting heavy.