I'm not doing maths & philosophy but I am doing maths & CS, and I think the maths portions are pretty similar (so I know what it's like not to do the applied that gets cut out).
To be honest, I'm perfectly happy doing it this way. I'm generally more interested in the pure stuff than applied, and while I am quite interested in probability and miss that stuff a bit, I enjoy the CS a lot and I'm glad I don't have to do as many differential equations and stuff.
It's also nice doing a joint degree as you get a chance to have a break from maths and do a problem sheet of something else for a bit.
The stuff you don't do is a lot of calculus, a bit of stats, dynamics, fourier series... if any of that is particularly appealing to you, go for straight maths. I have a friend who was on my course but switched to maths & stats last term because she wanted to do more calculus and statistics. But if you're not bothered about that stuff, go for the joint degree (and depending on your college you may have an opportunity to switch in the first year anyway if it's the wrong decision). Do consider what you're really interested in, though, as I know people here who really don't like the pure stuff and are all about applied, so if you turn out to be one of them you'd regret doing a joint degree. Or you might look at the work your pure maths friends are doing and think "thank frig I listened to LtCD and went for the joint degree!"
Philosophy is something you can pick up later, but will you really pick it up later at the level you'll have to opportunity to study it at at university? Will you care enough? And yeah, maths is harder to pick up later - but if you're on the joint degree and end up doing a PhD in maths or something, you'll easily be able to pick up the undergraduate level calculus that is missing from your repertoire as your basis in the really important stuff will be strong.
I'm sure some maths & phil people can come in and be a bit more specifically helpful, but that's my perspective as a joint maths person.