wow, youre worried about a 1070?? ive only just upgraded to a 970 lol. a 1070 will be fine for a long time. but why try and make it last 5 years? its easy to take out and swap over a card. you can always sell it on and finance a newer card with it if it does start to struggle.. rather than trying to stretch it 5 years.. go 2 or 3 and sell it while its still worth something. in 2/3 years time itll probably still be worth £200.... in 5 years time, sod all..lol... basically dont panic about trying to make it last 5 years.
but saying all that. a 4670k or a 4770k or a 4790k wont make much difference for gaming, theres really not much between them. most benchmarks have them near identical in games. yeh theres a few games that benefit from a better cpu, but not many. its the graphics card that does most of the work. unless you do a lot of encoding or something? and the 6700k isnt any quicker than the 4790k either.. in some games its actually a few frames slower...
if your budget is £650.... then forget mucking around with the cpu or any of that.. just buy a gtx 1080... job done. itll blow away any small performance increase you might get from the slightly faster cpus... it coincedentally sells for £650. must be fate lol. and itll be far more future proof than the 1070.. a quick and painless upgrade instead of rebuilding a whole computer..
and dont worry, the 4670k wont bottleneck the gtx1080. out of interest, have you overclocked it yet? theyre very nice overclockers and will happily sit at 4ghz minimum. so you could give your system a bit of a refresh by overclocking it if you havent already done so....and another potentially daft question, have you got an SSD? another relatively inexpensive upgrade thatll breathe a lot of life into an older system...