Salam aleikum wa rahmatu allah wa barakatuh....
Tomorrow in the general election in england is it halal to cast a vote in a non islamic country with non islamic laws or is it haram?
I have been told that to vote in a system that has no shariah law as it's legislation is haram? Is this right or is it wrong?

All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.

Participating in elections is among the issues of the Islamic policy formulation (Islamic politics) which are based on the rules of jurisprudence that relate to public benefit versus public evil. Whenever the benefits of participating in the elections outweigh its evils, participation preponderates, and whenever its evils outweigh its benefits, non-participation preponderates, or it may even be deemed obligatory (to refrain). This matter is determined by the Muslim scholars in each country, as they are more aware of the circumstances of their country. We mentioned in previous Fataawa that the basic principle in this regard is that it is forbidden to participate in the elections if the system of governance is not Islamic (does not conform with the Islamic Sharee‘ah) unless the knowledgeable scholars believe that participation is entailed by a Sharee‘ah-acceptable benefit. Shaykh Ibn ‘Uthaymeen issued a Fatwa for the Muslims in America to participate in the elections and vote for the candidate who supports the interests of the Muslims. Dr. Ahmad ibn ‘Abdur-Rahmaan Al-Qaadhi said in his book Thamaraat At-Tadween: “I asked our Shaykh whether the Muslims in America should participate in the elections in the US and vote for a candidate who supports the interests of the Muslims. He replied affirmatively without hesitation.” [End of Quote]

For more benefit on participating in elections, please refer to Fataawa 84199, 82201, and 81429.

Allah Knows best.