School trips, on public transport, will be well-managed, in an age-appropriate way. It's a really important thing for children to learn.
Agreed. When we were small, my mum used to take us up to London and we'd travel round by bus and tube. So when I was old enough to travel up on my own I knew how to use the tube and was quite confident about doing it. Children need to build up experience so it's no big deal when they come to do it themselves.
OP, one thing you could do, if she can't do it already, is teach your dd to say her name and home address, and the name and address of her school, and be able to repeat it to a person in uniform - tfl person, police, security staff - so that in the unlikely event of her becoming separated from her group she can be identified and the right people informed quickly.
(A friend and I got separated from our group in the BM once when we were about eight. We'd been dawdling and not paying attention. We found them ourselves without any alarms being raised. I still remember scuttling past the mummies wondering if we were going to be in trouble, but not scared about being lost.)