The flipside to this is you've said your insurance doesn't change much from one year to the next. So you know it will typically be about £500 and always due on the same date. It doesn't take hardly any effort at all to manage your finances that way; if you are in a position where you can't handle a recurring bill of only £500 once a year, then that's probably even more reason to spend a modicum of effort on sorting it out as you'll then have more money to handle such things. In other words, if your finances really 'just worked', you wouldn't bat an eyelid at a £500 bill, even if you'd somehow managed to forget about it.
Obviously it's your choice how you run things but I would strongly recommend moving away from using expensive finance unnecessarily. I also worry about people who run everything on monthly budgets in terms of what happens when they get surprises i.e. the genuine surprises that aren't recurring bills or covered by insurance.