Instructor isn’t an academic title – it’s just a person teaching a course.

Lecturer is like Assistant Professor, except lecturers will be on permanent contracts in some cases, as promotion and tenuring are independent in Britain.

Senior Lecturer and Reader are both roughly the equivalent of Associate Professor. Reader is generally considered to be a more prestigious position, but many universities are trying to re-define these things so that reader is a more research intensive position, and senior lecturer is more teaching intensive (although both contain a mix of teaching and research). The salary scale is identical for the two ranks.

Tutor is usually like undergraduate academic advisor, although there are also positions like admissions tutor for the faculty member in charge of the department’s undergraduate admissions.