distilling by any means in the uk isnt illegal but you have to have a licence
"Spirits Duty
Spirits Duty is payable on any spirits, or any mixture or combination of spirits with anything else, at a strength of more than 1.2% ABV.
If you want to produce spirits in the UK by distilling alcoholic liquids or by any other process - such as cracking ethylene gas - you will need:
a distiller’s licence
approval for your plant and process
to account for and pay Spirits Duty to HMRC
Spirits Duty: licensing and approval
In order to apply to HMRC for a distiller’s licence, you should make a written application and provide details of your plant and process. You should bear in mind that HMRC can decline an application if the largest still to be used has a capacity of less than 18 hectolitres.
You must apply for a distiller’s licence using a set format."
https://www.gov.uk/alcohol-duties