Cold pitching a venture capital (VC) firm can be more challenging than making a warm introduction, as you don't have the benefit of a mutual connection to help introduce you. However, it is not necessarily a "bad" thing to cold pitch a VC firm, as long as you are respectful of their time and are able to clearly articulate the value of your business and why it would be a good investment opportunity.
That being said, it is generally more effective to try to make a warm introduction if possible, as this can help to build rapport and establish a relationship with the VC firm or investor. If you are unable to make a warm introduction, be prepared to work harder to make a strong case for your business and to be persistent in your outreach efforts.
We'd recommend starting with our list of venture capital firms in the UK, that include details on how to cold pitch them, as well as application links.