UK ratings are:

U (equivalent to US “G”)
PG (equivalent to US “PG”)
12/12A (equivalent to US “PG-13;” from here on out, no one under the specified age is allowed in the theater or to purchase a copy, period - EXCEPT for 12A, which is theaters-only and a recent addition; under 12 requires adult accompaniment; 12A movies are all 12 on home video, so no one under 12 is allowed to buy them at all)
15 (kind of a split equivalent to either a hard “PG-13” or a soft “R,” depending on the movie)
18 (equivalent to a hard “R” or an “NC-17”)

So if a movie is rated R in the US, it might be either a 15 or 18 in the UK. And the other ratings aren’t 100% for sure analogs, either - some PG-13 US movies might be PG in the UK, some might be 12A/12, some might be 15.

The other important thing to note is that UK ratings are mandatory for all media - it’s illegal to distribute a movie unrated in the UK, whereas in the US it’s not a law at all, just generally difficult to market and distribute unless it’s digital or home video.