General immigration rules are largely found in 'Part 1: leave to enter or stay'.
When applying for any visa, check which you can apply for, what it costs, what it gives you, what you need to prove and what forms to use.
Beware that although each route has a suitability test, there is also a general suitability test for all routes called 'Part 9 grounds for refusal'. This should not be a problem if you are healthy, of good behaviour and stick to the visa rules. It includes matters mainly concerning public interest, criminality and immigration compliance:
deportation,
non conduciveness,
criminality,
sham marriages,
false representations,
customs or immigration breaches,
withholding information,
non admissibility to onward destination,
unpaid NHS debt or Home Office litigation costs,
purpose not in immigration rules,
wrong visa,
missing passport,
illness,
unaccompanied minors without consent,
changed circumstances,
changed purpose,
rough sleeping in UK,
ceasing to meet requirements,
main applicant's leave cancelled,
loss of sponsorship or endorsement,
failure to pass English test after pre-sessional course,
failure to study on course,
failure to work for sponsor,
change of job,
fall in salary.
Visas can be subject to bans on work, study and public funds, and can require police registration and medical inspection. Medicals are the norm for stays over 6 months.
You lose indefinite leave if you leave the UK for 2 years unless you have strong ties to the UK and are making it your permanent home.
You are subject to consideration of circumstances before readmission if you have leave to enter or remain of no more than 6 months and leave the common travel area (of UK, Ireland, Channel Island and Isle of Man).
You normally need a passport or entry clearance such as a visa to enter.
Leave can be extended, shortened or made subject to conditions.
Whilst you can ask an immigration officer to vary your leave on arrival they do not have to consider it and you might simply encourage them to cancel your leave on the basis it is not the real purpose of your visit.
Sponsors breaching undertakings to support entrants can be guilty of social security fraud.
Fiance(e)s or applicants for over 6 months leave need a TB certificate.
Evidence can be ignored if it fails verification (or if there is reasonable doubt even without verification for indefinite leave), and, although can be copies, must have contact details and certified translations to help verification.