A 980TI actually performs just a little shy of a 1070, better than a 1060.
A 1060 is more 980 territory (3GB variant a little worse than a 980, 6GB a bit better).
The minimum I can find a 1070 for in the UK is 340GBP, for a 150W TDPhttps://uk.pcpartpicker.com/product/4fyxFT/zotac-geforce-gtx-1070-8gb-mini-video-card-zt-p10700g-10m
A 980TI is a 250W TDP card, so 100W difference.
100W x 24 hours x 365 / 1,000 = 876 KwH - if used 100% load, 24/7 for a year.
The average cost of a KwH is right around 12 pence.
So your "additional" cost of running would be roughly 105 GBP annually...... if you ran at 100% load 24/7.
Since most peoples usage would only realistically be anywhere close to "100% load" a few hours a day at best, and add a couple more for 'averaging' out moderate use, say 6 hours a day 100% load (or 1/4 of the 100% load 24/7 cost).
A realistic, annual cost of running increase would be more in the 25GBP .... annually!
So yeah, a 980TI for 200GBP would totally be "worth it" IMO.
Right at 1070 performance, with 4-5 years worth of utility bills before you even spend the 140GBP difference in price via electricity.