Immigrationboards.com

I have been living in the Isle of Man since 2019, on a Tier 2 General Visa that was issued for 3 years. During this time I have not travelled abroad and have stayed on the island and within mainland UK. (I do not have a BRP as my visa vignette is for 3 years and also because it is not provided if the stay is in the Isle of Man).

Now after 2 years, if I move to mainland UK and start employment there, will this time period spent in the Isle of Man be counted towards the 5 years continuous period required for the ILR?

To the best of my knowledge, no.

The Isle of Man, Guernsey and Jersey are all separate jurisdictions from the UK and from each other for the purposes of immigration. So I doubt that you will be able to aggregate time across the jurisdictions for the purpose of immigration.

The UK Immigration Rules do not extend to the Isle of Man and the very first requirement for applying for ILR as a Skilled Worker (replacement for Tier 2 in the UK) is "The applicant must have spent a continuous period of 5 years in the UK.". The UK is defined in the Rules as just the "United Kingdom".

By contrast, the British Nationality Act 1981 specifically states that the United Kingdom includes the Isle of Man and the Channel Islands for the purposes of that law.

I would imagine that you will want to remain in the Isle of Man until you are naturalised. Once naturalised, you will be a British citizen even when naturalised on the Isle of Man (though your passport may be different to the passport of a person naturalised in the UK).