You should mention who you are travelling with. It's significant information, and UK authorities expect you to give them all relevant information. If there isn't a question on it specifically, there is I believe a space where you can add any other relevant information.
You should definitely explain the transfers to your husband's account. However a letter will not be sufficient - you need to include statements from your own accounts. You need to do this anyway, but especially you need to show the transfers coming from your husband's account and to your husband's account. Making that transfer wasn't necessary and probably wasn't helpful. Showing the money in your own account would have been just as good.