I don't know about workload for joint with economics, but I'm doing philosophy and english and you do seem to get a lot more work doing joint. But of course, I don't know how much reading you'll get for economics; maybe not as much as english?
You don't get many lectures for philosophy. You'll most likely have a two week timetable, so it's hard to work out per week, but in your first term for philosophy I can tell you that if the core modules stay the same as this year, you'll have four hours of lectures a week plus two hours of seminars every two weeks. The modules for first term philosophy are Self Mind and Body, Moral Philosophy and Logic.
It's generally fairly easy to transfer to one subject of your joint degree in your second year, or so I am told by my tutor. But if you decide you don't like philosophy in the first few weeks, it shouldn't be too much of a problem to catch up on missed work as you'll only have missed stuff from one economics module. I don't know about signing up to subsids though, it might be harder doing that after a few weeks as you'd have less choice.