As an asylum seeker you are generally not allowed to travel outside the country where your process is taking place. If you do so, your asylum procedure in Italy will end automatically because you are not taking part in the procedure and because the UK issued you a visa. Therefore you are not in need of protection by the Italian state anymore.
You can wait to receive a residence permit by Italy and then apply for a work visa to enter the UK. Another option would be to withdraw your asylum application and apply for a visa in a British agency abroad directly.
You can find information about work visa provided by the British government following this link: