I had this done about four years ago and like you was really scared before the op, never having had a general anaesthetic before.
But it was really really fine. I didn't have real pain after it, just discomfort but a lot of that was due to having two chest drains in (don't know if this is standard in the UK as I am abroad). Don't remember having much in the way of painkillers but the hospital kept giving me icepacks to put on my upper chest and scar for about 24 hours afterwards and I found that soothing.
For the first few days I found it hard to swallow liquid and food but found using a straw helped with drinks. I think it improved as the swelling in my throat went down.
I had a paralysed vocal chord afterwards, which can be a complication. The surgeon was adamant it hadn't been severed (no cure for this) and I was seeing a ENT for a while afterwards, who didn't do anything but just looked in and confirmed it still wasn't working. This was odd - it made me softly spoken and I couldn't sing more than about five notes. Then all of a sudden three months post op, the vocal chord recovered and I have no lasting effects whatsoever.
My scar looked quite crap after for a while but it was stitched not glued and I believe glue gives a better finish. I was also stupid enough to lift my then 16 month old the day after the op and quickly realised that didn't feel good on the scar! However now my scar is almost invisible. I used scar cream and moisturising cream on it for a while but actually I think what helped it fade most was wearing a necklace over the top of it (which I have been wearing for the past three years) as this really kept the sun off it (I only wear short necklaces now as I feel longer ones really draw attention to it). This summer I went out without the necklace and suddenly realised that the scar is really hard to spot nowadays.
Hope this really reassures you as I do remember the fear beforehand and hope I can allay those fears a bit. And on the plus side, I now have a far slimmer neck than before as my infected thyroid was quite hippo-like and actually my underactive thyroid has been so much better under control since. Very glad I had the op.
Good luck and get well soon.