Typically, UK censors care far less about sex and nudity than they do about violence, and this is reversed in the US. However, Top Gun was a bit of an odd one. It wasn't really strong enough to warrant a 15, but because there was no 12 rating at the time, it was felt the sex scene was too erotic and prolonged to contain at PG, and that there was a bit too much swearing (including one f-word). Hence the 15 rating, later reduced to a more sensible 12 (which is akin to PG-13 in the US).

The UK also has a history of lunatic film censorship, which I detail here, if you're interested. Thankfully this is a thing of the past.