Sexual assault is a much broader term. The law defines rape very specifically. From the Sexual Offences Act 2003:
"1 Rape
(1) A person (A) commits an offence if—
(a) he intentionally penetrates the vagina, anus or mouth of another person
(B) with his penis,
(b) B does not consent to the penetration, and
(c) A does not reasonably believe that B consents."
"2 Assault by penetration
(1) A person (A) commits an offence if—
(a) he intentionally penetrates the vagina or anus of another person (B)
with a part of his body or anything else,
(b) the penetration is sexual,
(c) B does not consent to the penetration, and
(d) A does not reasonably believe that B consents"
"3 Sexual assault
(1) A person (A) commits an offence if—
(a) he intentionally touches another person (B),
(b) the touching is sexual,
(c) B does not consent to the touching, and
(d) A does not reasonably believe that B consents"
Are they using "sexual assault" on legal documents or just when speaking to you?