An iMac would be a far better value purchase, yes - but only if you don't need it to be portable, obviously. You said you have your iPad and work laptop for that, though.
Because you're doing video editing, an i7 would actually be highly beneficial for you, as would an upgraded GPU (could someone actually confirm that macOS video editing software utilises GPU acceleration nowadays??)...
5K iMac
4.0 GHz i7 6700K (desktop-class)
4GB M395X (mobile-class)
2TB Fusion Drive
8GB RAM (upgrade later yourself if needed - no warranty void issues on iMacs)
£2,699
15" TB MBP
2.7 GHz i7 6820HQ (mobile-class)
2GB Radeon Pro 455 (mobile-class)
512GB SSD
16GB RAM (non-upgradeable)
£2,699
The 5K iMac build
benchmarks about 30-40% higher than the MBP, for the same price. You can also edit 4K footage natively on the iMac, and it still won't take up the full screen.
Video editing/rendering is one of very few tasks where benchmark numbers actually provide at least some relevance to real-world performance differences. The obvious downside of the iMac is that it's not portable.
In short, the 5K iMac offers better performance and value, but is only relevant if you don't need portability. It's also likely to be updated in the not-too-distant future, but that's pretty much always the case with tech.
Edit: One thing to flag - if you're writing/rendering/working with video files that are tens of gigabytes in file size, then you might suffer from an I/O bottleneck with the slow spinning HDD in the iMacs. Someone with more video editing knowledge might be able to advise on this.
The problem with Fusion Drives is that they're terrible for people who work with large files - in these cases they are bottlenecked to a speed achievable in the late 90s because of the old spinning drive they use. This graph perfectly illustrates this:
http://images.techhive.com/images/article/2012/11/fusionstress_filestoexternal-100015134-orig.png.
