Is Country Music making a comeback?

Actually, Newton-John's success is relative to which era you're focusing on. Early in her career, throughout the early 1970s, she was quite popular in Australia and the UK but struggled for success in the US, where she only had one minor top 40 hit in "If Not For You." This all changed, however, in the mid-'70s; she finally had a major commercial breakthrough in America at the turn of 1974 with "Let Me Be There," while her career suffered in Australia after "Have You Never Been Mellow" and fizzled out in the UK after her Eurovision entry "Long Live Love." Except for the fluke success of "Sam" in the UK, Newton-John's career did not resurface outside of the US until she starred in Grease, by which point she began abandoning her country roots for more of a hard pop edge.
To be honest, country has not had a true fanbase in the UK since the mid-'80s; it hasn't just been the 2010s that the genre has failed to crack the British market. The difference is more that the '90s and 2000s at least produced a few rare exceptions, such as "Achy Breaky Heart," Shania Twain, and Taylor Swift's "Love Story," whereas absolutely nothing from the 2010s charted anywhere significant in the UK.

I believe the two major factors for country's decline in the UK are generational divide and industrial evolution. Back in the '60s and '70s, country music in general had much closer ties to pop and rock, probably because both genres back then were woodsier and not yet influenced by synthesizers or hip hop. For example, Glen Campbell's "Wichita Lineman" was written by Jimmy Webb, who had also penned "MacArthur Park" for the British singer Richard Harris, and frankly you can easily hear the similarities between the two songs' styles, even though the former has more of a rural American subject matter. Additionally, rock music, for the most part, still often had strong ties to country music, which it technically stemmed from in the first place. For instance, Creedence Clearwater Revival, though more popular in the US, still scored a few notable hits in the UK.

Things began to change dramatically, however, when the Generation X-backed new wave and synthpop movements of the late '70s and early '80s started to revolutionize the music industry as a whole. Not only were the Brits turning to more futuristic and polished music than ever before, but in the US, the country industry entered a commercial dark age that lasted from the full establishment of MTV in the mid-'80s all the way to the end of the decade. From 1984 to 1991, there wasn't a single legit country hit single on the Billboard Hot 100, as the twangiest mainstream music ever got was bands and artists like ZZ Top, John Mellencamp, and Bob Seger. Country music during this period was nothing but a minor niche genre, much like rock music has been this decade, though possibly even less so. From what I understand, the stodgy, family values conservative consumers who had largely backed country music for the Baby Boomer Generation were instead migrating to the rapidly growing Contemporary Christian music scene. In this climate, obviously, country didn't stand a ghost of a chance at crossing over to the UK, so the scene that singers like Dolly Parton, Glen Campbell, Tammy Wynette, Johnny Cash, Ray Stevens, Olivia Newton-John, and Kenny Rogers helped cultivate during the '60s and '70s was more or less completely extinguished after "Islands in the Stream" from 1983.

Country music gradually became a serious deal again during the 1990s, but not in a way that was conducive to international success. Singers like Garth Brooks, though absent on the Hot 100, significantly revitalized the country industry for the non-top 40 crowd, which helped pave the way for numerous future legends like Tim McGraw and Shania Twain to eventually make more of a foray into top 40 success. Despite this, however, country's general audience was still much more segregated from the pop crowd than it had been in the '60s and '70s, so even in the US, you almost never saw monster #1 hits on the Billboard Hot 100 like "The Streak" and "Rhinestone Cowboy" were in the 1970s. Although country songs have been a constant presence on the bottom half of the Billboard Hot 100 since 1999, it's still overwhelmingly common to come across people here whose music taste is "anything except country," something that wouldn't have been nearly as true back in the '60s and '70s, when it was more integrated into the general mainstream. Since country music has only ever been big in the UK as part of the mainstream fabric as opposed to a scene with a robust niche following, it has not been able to consistently survive on the UK Singles Chart for decades.

It should be worth noting that the few country songs that have cracked the UK market are strictly novelties, like "Achy Breaky Heart," or whose country influences are watered down in favour of more pop tendencies, as was the case with The Mavericks' "Dance the Night Away," Texas' "I Don't Want a Lover," and Taylor Swift's "Love Story." Even with Shania Twain's Come on Over, the two most country singles from that album, "That Don't Impress Me Much" and "Don't Be Stupid (You Know I Love You)," were specifically remixed into dance/electronic songs for mainstream airplay, while the rest of the hits were pop enough to blend in with your usual pop-rock and adult contemporary songs of the day. In other words, all of the country songs that got popular in the UK after 1983 were successful specifically because they appealed to listeners who weren't fans of country in the first place.

Justin Timberlake and Chris Stapleton's "Say Something" will likely be the biggest "crossover hit this decade in the UK for the same reason. Its main star is already super popular in the UK, and is largely a regular pop song that happens to feature a country singer, as well as some traditional Chinese musical influences, of all things. If the song didn't have Stapleton singing the second verse, it frankly wouldn't count as a country song to any real degree whatsoever.