I was the owner of a limited company which made a profit and gave me a dividend of £18,000 from it. However, I couldn't file my annual returns and the company got dissolved. As a result, I did not pay any tax on the company dividend. I was applying for a visa and was informed that I have to pay tax on the £18,000 dividend or my visa application may be turned down. I am not trading through that business anymore, and am employed with a company at the moment. How can I pay the tax on this £18,000 dividend when I don't have a company any longer?

Even if you don't have a company any more, the fact is that you did receive an £18,000 dividend from the company, so you personally have an income tax liability. If you have £18,000 income in your 'personal' pocket, there is personal income tax to pay. It doesn't make a difference where it came from.