That's a very difficult decision, even more so because we don't know how much the 4070 will be in pounds. Will it be £500 or more? And since that's the MSRP, how much will it actually be?
At the same time, it looks like it will perform like a 3080 and, therefore, like a 6800XT, but with much better RT results.
I don't know, man. If that £520 deal for a 6800XT will last until the release of the 4070, I'd tell you to wait so you have all the details and can make a better decision, but if that deal won't last long... well, it's up to you.
Sorry I can't help, but I don't think there's a right or wrong option here.
Please excuse my bad English.
Currently gaming on a PC with an i5-4670k@stock (for now), 16Gb RAM 1600 MHz and a GTX 1070
Steam / Live / NNID : jonxiquet Add me if you want, but I'm a single player gamer.