In general, you would typically be considered a resident from day one, based on your intent to, in fact, reside in the country. Thresholds like this 180-day criterion are used to define who is a resident for certain purposes but it does not mean you are only considered a resident after that time. For example, if you move in on the first day of the year, you will be taxed as a resident for the whole year, and not only for the last six months.
Similarly, in the EU, non-EU nationals typically need a visa to stay more than three or six months but very often they need to have the proper kind of visa/permit when first entering the country and not only after a three or six-month period (although there are some exceptions as well).
In the case of the UK, the language on the gov.uk website clearly implies that your residency status (for tax purposes) is assessed based on the whole year (April to April). Note that the 183-day presence is not the only test, having your only home in the UK is also enough, even if you spend a lot of time abroad.
HMRC publishes extensive guidance on the criteria and in particular on how they treat “split years“ (the years in which you move in or out of the UK) but my understanding is that, unless you have a complex situation with interests in various countries, you are definitely treated as a resident when you take up employment.
As far as EU law is concerned, you become a permanent resident, with some additional rights, after working for five years in the country (or otherwise making use of your freedom of movement rights). The main difference is that if you lose your job in the first five years and are unable to find another one or become destitute, you could be asked to leave and go back to your country of origin. Once you became a permanent resident, you can choose to remain in the country and get welfare or disability benefits on a par with citizens. See Europa.eu for the basics on this distinction.
It's obviously not something you plan or wish for but it's a major legal difference and does give you more options if you would run, e.g., into serious health issues.
Importantly, permanent residence as an EU citizen is not something you need to apply for or be explicitly granted. Legally, you are automatically considered a permanent resident as soon as you fulfil the conditions. You can however apply for a permanent residence card that confirms and documents your status (or, before that, while you are still a non-permanent resident, for a regular residence card which is also optional).