I'm going to go against the grain here and say YABU. The praying part is neither here nor there IMO - bowing your head and keeping quiet is simple enough to do and no-one can tell if you're praying or not.
However, I think traditional assemblies - with hymns, the Lord's Prayer etc is a crucial part of British culture and history and plays a key role in understand the world around us (phraseology, the House of Lords, the role of the monarchy, Shakespeare etc). I think learning about the Bible, learning prayers, singing hymns provides children with an education about British culture.
I think it's important to know what someone means if they comment (for example, in this weather) that they'll have to start building an Ark, or, if you ask where someone is and receive the reply "am I my brother's keeper", you'll know it's a reference to Cain and Abel.
One could argue that you could be taught these things in the context of an RE lesson, but I don't think that's true. I think the ritual, the hymns and the prayers provide cultural references that a one-off class about Christianity couldn't cover.
Of course, children can learn this in church, but if the parents don't go to church then that child is losing out on learning about Britain's cultural heritage.
I think, personally, one is socially disadvantaged if, for example, you attend a funeral in a church or go to a religious wedding and don't know how to follow the order of service or don't know the words to the Lord's Prayer. I suppose it does depend on what circles you move in, but I'm grateful for the daily religious worship we had in my (independent) school and I'm making sure DD knows about Britain's cultural heritage too (by taking her to church etc).
I don't think the secularisation of Britain would be particularly beneficial - Christianity is still crucial to British culture, its laws and they way in which the country is governed. Further, the United States is ostensibly secular yet the role of right-wing Christians is terrifying.
I do concede, though, that I am quite establishment-oriented and traditional (in some ways - I'm also a single parent in a gay relationship so quite unconventional in others!
)