The position of EEA nationals is special. They do not require leave to enter the UK because they have an enforceable Community right of admission. Section 7(1) of the Immigration Act 1988 and Regulation 11 of the Immigration (EEA) Regulations 2006 SI No.1003 which grants an automatic right of admission on production of a valid national identity card or passport.
Whether an EEA national is a person subject to immigration control depends on whether they are exercising an enforceable European Community right (Immigration Act 1988, s.7(1)), i.e. whether they have a right to reside. Their right to reside derives mainly from Directive 2004/38/EC ("the Directive"). This has been given domestic effect in the UK by the Immigration (EEA) Regulations 2006 SI No. 1003. However, advisors should check the Directive and the regulations on problematic cases because they are not identical.
An EEA national not exercising a right of residence has no right to remain in the UK and, accordingly, requires leave to remain. These EEA nationals are persons subject to immigration control: see Abdi & Ismail v Barnet LBC  EWCA Civ 383,  HLR 23, 6 April 2006.
In all cases, where an EEA national has a right of residence he/she will not be a person subject to immigration control.
There are 3 classes of right to reside provided by the Immigration (EEA) Regulations 2006:
Initial right of residence pursuant to Regulation 13 not exceeding 3 months, valid identification card or passport, must not become an unreasonable burden on the social assistance system of the UK;
Extended right of residence, pursuant to Regulation 14 - available only to qualified persons;
Permanent right of residence pursuant to Regulation 15 - principally available to EEA nationals who have resided in the UK in accordance with the Regulations for a continuous period of 5 years and to workers/self-employed persons who have ceased activity.