To me the online option is a recent innovation (I went though the theory courses a long time ago), but I know quite a few people who have done it that way and swear by it.
I did my Day Skipper as a combined course 4 days of theory and then straight out on the boat for the practical which is another option.
I think that there are some RYA schools whose instructors explain the theory as you go on the practical, coupled with some paperwork exercises in the evenings on the boat. I have come across this on the net but can't remember the links...
Either way I would get the theory in some way and not just try to wing it.
The Day Skipper practical is an instructor assessed course and if the instructor passes you then he (or she) is saying that you have shown that level of competence and have shown that you can apply what is taught in the theory.
Don't underestimate the knowledge needed for practical. I had a crewman who had sailed over 2000 miles with me. I paid for him to do the combined theory and practical.
He passed the theory and then failed the practical for some reason which I was never able to get him to explain to me.
He is not the only person that I know to have done this as one of my own class failed the practical after passing the theory (just), after nine days with the guy I would have had to agree with the instructor's decision.
Good luck with your start to "the paper-chase" whichever route
you decide to take.