I would recommend OnePlus Two. It was advertised as a "flagship killer" mainly because it was comparable to many big brand flagships that came out the same year such as Samsung Galaxy S6. In fact, in some aspects, it was even better than the S6 - most notably in the battery life department. The S6 has a pathetic battery (2550mAh). OnePlus Two on the other hand has a much larger 3300mAh battery. However, do keep in mind that battery life of a phone depends on the user; some people will drain its life in a matter of few hours while light-users can keep it going for the whole day, if not more.
As for its camera, it's got a 13MP sensor and an f/2.0 aperture. This results in pretty decent photos and for the average user, that's completely fine because they won't really be able to tell any clear differences between a photo taken by the OnePlus Two or another flagship phone. It's important to note that resolution (in this case 13MP) only matters partially. There are other aspects to consider, such as a wide aperture and how good the sensor is. This means that the Nokia Lumia 1020 which boasted a 41MP camera didn't actually end up having the best camera in a smartphone because it didn't have a wide aperture and the software within wasn't good enough to use that 41MP to its full potential anyway. The best camera currently in a phone is almost definitely the new Samsung Galaxy S7 or the S7 Edge. The iPhone 6S/+ is a close competitor as well. You can expect the Apple and Samsung flagships to completely dominate the smartphone cameras for years to come.
You seem to be an Android fan so OnePlus Two is a really good choice in terms of OS. It has OxygenOS which is virtually the same as stock Android, with a few added features here and there. It's very similar to the Nexus line. You'll want to root this phone immediately once you get it and Google encourages users to root their phones anyway, I think.
The only problem is I don't know if there are any SIM plans for it. It's a really cheap phone for what its worth - on the website, it's £249 for the 64GB version. It's much cheaper than any other flagships I've heard of. Now that I think about it, this might actually be a huge deterrent for some aha I know I'd be quite phased if I had to pay that much up front. You could always call Three or Vodafone and ask. Hope that helps!