'Psychosis' Classification

Looking on the BBFC website, and researching modern thriller films (Scorese, Nolan). The majority of these are either 12 of 15 certificate. Examples include:
Inception

Shutter Island

Memento

Any bold text below (12/15 certified descriptions) shows content likely to be seen if 'Psychosis' were a full feature (only the 'oldest' will be in bold, ie a 15 may be in bold but not the 12 counterpart):
The BBFC description for a 12/12A film is
"Aggressive discriminatory language or behaviour is unlikely to be acceptable unless clearly condemned"
"Any misuse of drugs must be infrequent and should not be glamorised or give instructional detail"
"Moderate physical and psychological threat may be permitted, provided disturbing sequences are not frequent or sustained."
"Dangerous behaviour (for example, hanging, suicide and self-harming) should not dwell on detail which could be copied, or appear pain or harm free."
"Moderate language is allowed. The use of strong language (for example, ‘f*ck’) must be infrequent"
"Nudity is allowed, but in a sexual context must be brief and discreet."
"Sexual activity may be briefly and discreetly portrayed. Sex references should not go beyond what is suitable for young teenagers"
"Mature themes are acceptable, but their treatment must be suitable for young teenagers"
"Moderate violence is allowed but should not dwell on detail. There should be no emphasis on injuries or blood, but occasional gory moments may be permitted if justified by the context."


And a 15 on the BBFC website constitutes as: 
"The work as a whole must not endorse discriminatory language or behaviour."
"Drug taking may be shown but the film as a whole must not promote or encourage drug misuse. The misuse of easily accessible and highly dangerous substances (for example, aerosols or solvents) is unlikely to be acceptable."
"Strong threat and menace are permitted unless sadistic or sexualised."
"Dangerous behaviour (for example, hanging, suicide and self-harming) should not dwell on detail which could be copied. Easily accessible weapons should not be glamorised."
"There may be frequent use of strong language (for example, ‘f*ck’). The strongest terms may be acceptable if justified by the context. Aggressive or repeated use of the strongest language is unlikely to be acceptable."
"Nudity may be allowed in a sexual context but without strong detail."
"Sexual activity may be portrayed without strong detail. There may be strong verbal references to sexual behaviour, but the strongest references are unlikely  to be acceptable unless justified by context."
"No theme is prohibited"
"Violence may be strong but should not dwell on the infliction of pain or injury. The strongest gory images are unlikely to be acceptable. Strong sadistic or sexualised violence is also unlikely to be acceptable."


All descriptions gathered from the relevant page on the BBFC website.


Although it would likely feature would feature elements from a 12 film (language, drug use, sex/ sex references) or below (no nudity), certain elements would raise the film to a 15 (threat, violence, 'dangerous behaviour'), if the full feature were to be rated by the BBFC.