I hold an Indian passport and am living and working in Ireland with a valid work permit.
Do I need a visa to go to the UK?
Do they have border control?
You are an Indian national (or other nationality classed as a 'visa-national', e.g., Chinese, Algerian, Zambian, and so on) who has a work permit from the Republic of Ireland and want to visit the UK. It means you are a visa national and have correctly observed that the ROI is part of the Common Travel Area.
You WILL require an entry clearance (visa) prior to leaving the ROI, you cannot apply for leave to enter upon arrival. Your work permit will be quite useful in your application, but is not, of itself, recognized by the UK as a viable travel document.
PERSONS ENTERING FROM THE REPUBLIC OF IRELAND WHO REQUIRE LEAVE TO ENTER BY VIRTUE OF ARTICLE 3 OF THE IMMIGRATION (CONTROL OF ENTRY THROUGH REPUBLIC OF IRELAND) ORDER 1972(AS AMENDED)
- 1.1. Persons in transit through the Republic
- 1.2. Visa nationals who are not in possession of a valid United Kingdom entry clearance
- 1.3. Persons who entered the Irish Republic unlawfully from outside the common travel area
- 1.4. Persons who entered the Irish Republic after entering the United Kingdom or Islands unlawfully
- 1.5. Persons whose leave to enter or remain expired before leaving the United Kingdom for the Irish Republic
- 1.6. Persons whose exclusion has been deemed to be conducive to the public good
- 1.7. Removal of persons in respect of whom the Secretary of State has personally given exclusion directions who are identified within the United Kingdom after arrival from the Republic of Ireland
Your case is (1.2).
For your other question about border controls and (implicitly) can you benefit from the arrangements in place... There are limited border controls within the Common Travel Area for a lot of different reasons, but if you try to benefit from them you will become vulnerable to removal as an illegal entrant if you get caught. The UK police will also tell An Garda Síochána who may consider revoking your permit because you have breached conditions of leave (it should have been issued to you in a brochure along with your work permit).
You can apply for entry clearance on the net. You may not have the opportunity to present your entry clearance to an Immigration Officer upon arrival in the UK, and even if you go and find one on duty, they will normally refuse to stamp your passport. This is standard practice and does not affect your legal status.
Don't forget to get health insurance.
NOTE: Ignore what you may find in forums and discussion groups. The IDI's are here. The IDI's do not have legal standing (they are instructions only), however, this particular instruction was derived from Paragraph 24 of the Immigration Rules, which is enforceable under law (along with the Orders in Council of 1972).
Where in the UK? There is no physical border control between Northern Ireland and Ireland AFAIR. I don't know if you're legally allowed do it, but you might be able to actually do it.