From Wiki

In order for a person to obtain university education in the United Kingdom, it is necessary to meet requirements concerning both citizenship and 3 years residence. Being a British citizen, on its own, isn't enough. Similarly, 3 years residence in the United Kingdom is not enough.

The actual rules themselves are complex. The information below is a summary only and there are additional conditions and exemptions. Full details are given by UKCISA

Rules, and the application of rules, may be slightly different in England, Scotland, Wales and Northern Ireland.

The issue of overseas fees can in some circumstances affect further education (A-Levels in colleges) but this is not usually an issue for British citizens.

Citizenship / Immigration status

You must meet ONE of the following requirements:

• British citizen, OR
• Permanent Immigration status in the United Kingdom (Indefinite Leave to Remain, Right of Abode, or Permanent Residence under the EEA immigration regulations); OR
• Citizenship of another European Economic Area (EEA) nation or Switzerland

Residence requirement

• The residence requirement is 3 years ordinary residence in the United Kingdom, Channel Islands, Isle of Man, another EEA state, or Switzerland.
• Residence for the purpose of receiving full time university education does not normally count.

'Ordinary Residence

• Ordinary residence is not the same as physical presence.
• It is better described as where your "home" is
• If you have lived for a long time in the UK and are absent for a short term period, you may well remain "ordinarily resident"
• There is a lot of case law on the subject. Ordinary Residence - UKCISA