Can I stay in the UK for six more months if I stay for a few days in another country?

Question

My six months stay in the UK will expire on the 30th of December. EU citizen

Answer 1

Unless you have the right of abode in the UK, it is very likely you will be turned away at the border.
Staying in the UK is no longer a right for EU citizens and the UK does not allow people to make their home in the country unless they apply for the right paperwork before coming over.

And yes, they do ask how long you plan to stay if you get interviewed by an immigration officer.

In other words, as posted as a comment by Traveller:

Why do you want to return so soon after such an extended stay? UK Immigration Rule V4.2(b) prohibits visitors from living in the UK for extended periods through frequent or successive visits, or making the UK their main home. As @Willeke’s answer states, unless you have settled or pre-settled status it’s highly likely you would be denied entry (which would probably make future visits more difficult), or even banned if Immigration officials found that you have breached any Rules pertaining to visitors eg by working whilst in the UK as a visitor (that includes remote working too).

Answer 2

If you were to fly to the UK directly, you'd most likely get rejected at the border as correctly pointed out by @Willeke. The immigration officers are likely to suspect you're trying to live in the UK without a proper residency visa. However as an EU citizen you have an alternative route which is completely legal and which allows you to avoid seeing a UK immigration officer.

1. Book a flight to the UK via the Republic of Ireland or a flight to the Republic of Ireland followed by a ferry trip to the UK.
2. On arrival to Ireland, you're unquestionably entitled to enter as an EU citizen.
3. There are no routine immigration checks on flights/ferries to the UK and no requirement to otherwise report one's presence to the government
4. As per the latest CTA rules, you're entitled to a fresh 6 month stay if entering the UK via Ireland:

Those entering the UK from 1 January 2021 are entitled to 6 months of deemed leave, or 2 months of deemed leave where they have previously visited the UK on the basis of deemed leave (including before 1 January 2021) and have not left the CTA in the meantime.

5. There are no routine immigration checks on flights leaving the UK, so you may take a direct flight on your way home. Even if you do encounter an immigration check, there's nothing to worry about as you haven't violated any laws.

Needless to say, make sure you don't overstay, take up employment or otherwise violate UK immigration laws during your current or subsequent stays. And keep in mind you might have to start paying UK tax on your income due to your length of stay.