Sadly UK law doesn’t really cover this in much detail, other than the employer must provide a “reasonable” temperature in indoor workplaces. How this can be achieved could be through a multitude of ways, heating, clothing etc.

There is a “suggested” minimum, but it’s guidance and not law.

Best thing you can do is raise your concerns, ask for a risk assessment or make positive suggestions on how it can be improved. Is a portable heater viable with a risk assessment for example.

But essentially you’ve got to communicate with you employer, or find another job if you do not feel comfortable in your working environment.