I'm so sorry for your loss 
My advice would be to keep a very close eye on how you're feeling, take all the painkillers you can and if you have too much pain then go to the EPU straight away and don't rule out surgical management fit medical isn't progressing as expected.
I had a MMC at 12 weeks last winter, foetus developed to 8 weeks so wasn't that big but the sac and other stuff had all kept growing so it was effectively a 12 week loss. I was adamant I didn't want surgery and needed time to process what was happening and to be at home with my husband near me. I used the cervix dilating pills twice, a couple of days apart, and bled quite a lot but I knew it wasn't enough and the pain was incredible. This is unusual as the pills should work and it shouldn't be that bad. You obviously don't know what to expect when you haven't done it before but I'm glad I took all the painkillers they offered me, and then made a big fuss as I didn't feel it was progressing right and ended up having surgery - an MVA which is like an ERPC but under a local.
I don't at all want to frighten you and I'm sure it'll work fine for you and I hope it'll go as easily as these things can. But 12 weeks is quite far along for medical and I wish I'd been better prepared for what to expect and what not to expect. It turned out I'd never have been able to pass it naturally so surgery was the only option and as much as I'd feared it it was a blessed relief when I went in 5 days after we'd had the scan. The care was incredible and DH was with me up till theatre and waiting for me when I came out, and the whole thing tool about 30/40 mins and we went home straight after.
Have you looked on the practical tips for mc thread? I found that a complete life saver.
I'm so very sorry you're going through this. I hope you have people looking after you. There's loads of wisdom and support on MN. Bug your EPU as much as you need to. Do anything and everything that makes you feel as alright as you can - sleeping, crying, ranting, reading, walking, eating, not eating, getting drunk, sticking to water, seeing people, hiding out at home. If you're in too much pain make a big fuss, no one gets medals for suffering! And take more time off work than you need.