Arent they the same performance but the 480 cheaper ?
More or less, yes. In general, they're about equal in DX11 games (each is better in some), but the 480 is better in DX12 games (sometimes substantially). And the 480 is generally cheaper, at least if you're buying the GPU itself.
This has the gtx substantially higher... But it also doesn't have the 1080 at the top so not sure what it's worth
http://www.videocardbenchmark.net/high_end_gpus.html
The 970 is £10 cheaper at the place where I'm getting the rest of my build
That's one synthetic (not a real game) benchmark; if you look at RX 480 reviews that compare it with the 970, 980, 1070, Radeon R9 390, etc., such as
this one, you'll see that in actual games, it varies, but on average the RX 480 is even in DirectX 11 and somewhat better in DirectX 12, including results such as 40% better in Hitman and 15% better in Total War: Warhammer, at 1920x1080 where the 970 is closest. The average of the DirectX 12 games they review is 19% in favor of the 480; you'll probably see a similar spread on other reviews.
Currently, most games are DirectX 11, but especially as Windows 10 adoption picks up (and it's already over 40% of the PC gaming market), more and more games will support DirectX 12, and generally they'll perform better with it. In other words, for today's games they're essentially equal, but for tomorrow's games the RX 480 will be better, by enough to make the difference between playable or not playable at some point. The difference is due to the hardware design, so it's not something nVIDIA could equalize with better drivers.
So I'd probably go with the RX 480 if the difference is only 10 pounds. Over the next year, it won't really make much difference, but 3-4 years down the line, it could have a noticeable longevity impact. On the other hand, if you've already ordered, no reason to cancel; the 970 is still able to handle pretty much anything you throw at it without a problem and would serve you well for a good amount of time as well.