wanted an excuse not to have to wear uncomfortable clothes and make uncomfortable small talk for six hours. I find social occasions like this a trial as I'm not good with people.
Poor you! This does sound like an ordeal, for you. So, how about wearing smart but comfy clothes, and getting MN to help you through the day. 1st tip: break the day down into smaller parts:
Meeting before the service - take along a present for the baby and some flowers for the mum. Smile and thank them for inviting you (!) - yes, I know they didn't actually, but the thought was there.
The service - you never know, you may enjoy it. Chance to be in a beautiful building hundreds of years old, with windows and carvings and music that people have created to make it a special place.
The social bit afterwards - have a few packs of chocolate buttons and a bottle of bubble mixture. I always found that at painful family gatherings, you can usually entertain a small child and have fun. Then if it is a do at someone's house, head for the kitchen to see if you can help out. Other people who don't feel comfortable in social settings will be there so you can meet a kindred spirit and garner some brownie points which might come in useful, in future. (My alternative strategy is to find Great Grannie in the corner. She is sometimes a wonderful source of entertainment.)
Alternatively, just smile and nod lots, and enjoy a few glasses of wine, and do some people watching.