Is Friday Prayer obligatory for a Muslim residing in a non-Islamic country such as the United Kingdom? And is it sufficient to perform it once every three weeks?

It is not a condition for the obligation of the Friday prayer that it be performed in a Muslim country or under a caliphate. Rather, it becomes obligatory wherever the required number of people (three men or more) is available. The notion that performing the Friday prayer every three weeks is sufficient is invalid by scholarly consensus. Whoever misses three Friday prayers out of negligence will have their heart sealed, and whoever habitually misses Friday prayers will be among the heedless. What is obligatory is repentance and maintaining the performance of the Friday prayer every week.