Will the UK version of Millionaire return in 2018?

I doubt it. Thinking in anniversaries is a fan thing, not a business thing. Expensive primetime quiz shows just aren't popular in the UK right now, and I don't think they'll make a comeback anytime soon. The failure of "500 Questions" adds to that. Additionally, the show only ended in 2014. 4 years isn't really enough to announce "the epic comeback of a television classic".
And last but not least, who'll replace Chris? Jonathan Ross? Jimmy Carr? Mel and Sue?

From what I've seen with other remakes of classic U.K. Gameshows, it tends to be around 5-10 years or so after the original run has ended.
Shows like Family Fortunes and Mr. and Mrs. have both been remade successfully, albeit as 'All-Star' versions, the way Millionaire finished. However, unlike Millionaire these shows didn't have a lot of provenance, so quickly fell out of the public's minds after the end of the original run. Simply because of how influential Millionaire has been on the British public, it will take a very long time for people to forget about it. (I still hear my teachers, and even my friends, say things like 'Is that your Final Answer?' And 'Can I phone a friend?'. Millionaire made an impact on even the younger generation, born after 1998 and indeed Judith Keppel's win in 2000 (2 days after I was born, coincidentally), and is still incredibly well known 2 years after it's end.
I would be very surprised if Millionaire is never remade in some way, but I think 2018 will be quite too soon to get people interested again. There could well be a one off clip show, like Chris' Final Answer to commemorate 20 years, but a remade series of the show probably wouldn't happen for an anniversary. It would most likely happen when there's little or no gameshow competition to get people into the format.
When it debuted, Millionaire changed the gameshow world forever, and if it were to be successful properly again, it would need to regain and then keep that 'shoutability' it was so famous for.