Identify which item carries the label
A short marketing clip and a full feature are different works for classification purposes. The BBFC’s trailer resource gives the example that a trailer may be classified 12A while the advertised film is 18. If the only rating you have seen belongs to the preview, it has not answered the question about taking a child to the complete film.
Consider what the edit leaves out
A trailer selects particular images, dialogue and music to sell the film. The absence of an issue from those minutes does not establish its absence from a two-hour feature. Equally, an intense sequence in a trailer may be presented without the surrounding narrative that helps explain it in the full film. These observations concern how an edited promotion works, not a claim to have watched the unreleased feature.
Read the feature’s specific advice
Look up the full title and release version, then read the age rating and content advice associated with that feature. Avoid selecting a similarly named trailer entry in a search result. When planning a cinema visit, check the cinema’s actual film listing as well. A preview playing before another film does not establish that the promoted film has the same admission category.
Use the trailer as one source of expectations
The preview can help identify genre, actors or a premise, but it should not substitute for the classification record. If a child is particularly sensitive to threat, read the relevant advice before booking rather than deciding solely from familiar characters in the trailer. Keep the promotional impression and the supported classification information distinct in the final viewing decision.
Questions, answered
Can a 12A trailer promote an 18-rated film?
The BBFC explicitly gives that as a possible combination.
Is the trailer automatically rated exactly like the feature?
No. Its own content is assessed.