I would watch peoples side by side reviews and go based on them:
You'll find a 3080 is considerably better than a 3070 especially in RT (which is in the majority of games now and will be the norm going forward)
I use my 3080 for both 4k and 3440x1440 and whilst the 3080 absolutely smashes games at these res. with ease, especially if using dlss/FSR, however, you are going to struggle with RT going forward, several titles out there now where even a 3090/ti is struggling. If you don't care about RT or are happy to reduce settings or turn it off, a 3080 is a much safer bet, wouldn't waste an extra £400+ on the 3080 12gb vram model given it shows very little to no benefit in 99% of games, maybe in another 2-3 years, it might be more beneficial in a variety of games (unless direct storage takes off) but then by that time, you'll have low range gpus destroying the current flagship devices.
Also, your cpu will hold back both the 3070 and 3080 in the 0.1 and 1% lows, I had a highly clocked 2600 and went to a stock 5600x, the difference was night and day especially in cpu heavy games like cp 2077, even at 4k.
Ultimately at this stage, you would be better of waiting for the next lot of gpus, you'll be getting 3090/ti performance for half the money.