Much better? Not so sure how that works out, to be honest. They're obviously vastly superior in midfield but on paper Lingard, Sterling, Alli and Kane is better than Mandzukic, Kramaric, Rebic and Perisic. Kane is the only bonafide world class player in either attack and definitely better than the (still very good) Mandzukic, I don't think many would argue otherwise. Then Perisic is probably better than Lingard, although both have a similar goals/assists to games, with Lingard playing for a better team but in an arguably superior division - (Lingard scored 8 and assisted 5, Perisic scored 11 and assisted 8 but started way more games.)
Sterling is obviously better than Rebic too .. 18 goals and 11 assists in the Prem last season whilst Rebic managed 6 and 2. Alli managed 9 goals and 10 assists vs Kramaric's 13 goals and 6 assists, so again pretty similar. Attacking wise I'd say England definitely have the edge, Sterling & Kane both put up numbers that I don't think any of Croatia's attackers could manage in the Prem. Obviously, actually doing it at the tournament is a different thing entirely, but it's not as if Croatia's forwards have been pulling up any trees in the knockouts either.
We have better fullbacks too (Ashley Young/Rose are definitely better than Strinic), Vrsaljko is a very good right back (though could be injured vs England?) but Trippier is having an incredible tournament and was also very good for us last season, going forward as a wingback he's a really effective player with world class delivery. Neither of us have the best central defenders.. but give me a back 3 of Walker, Maguire and Stones over Lovren and Vida any day of the week.
Don't think there's much in it when you look at the substitutes either. England can bring a 20+ a season striker in Vardy and a really dangerous player in Rashford, whilst Croatia have the likes of Kovacic or Pjaca. Like I said, the likes of Modric, Rakitic and Brozovic are vastly superior to England's midfield options but in every other area of the field I think England have the edge. Croatia are a really, really good side who have shown some fantastic spirit to win two penalty shootouts, but England have also been pretty good and whilst it's not the golden generation we do have some really decent players who have had very good seasons in a tough league, just needed to be managed right.