It's not so much that his ashes are being put in Westminster Abbey, it's that there will be a "Service of Thanksgiving". Thanking who?
Here's a man who spent his life as an atheist, but now he's dead the god-squad muscle in. Giving thanks to a entity he didn't believe in, and co-opting their mythology onto his science work, "together to seek to answer the great questions of the mystery of life and of the universe".
I've no problem with people choosing to have religious services on their death, if that's what they believed and wished. I'm fine if it is all for the benefit of friends of family. But having these performances upon the death of those who specifically refuted them seems extremely disrespectful.