This is entirely possible. You need to be careful that you are not transgressing your employment terms (as laid down in your employment contract and/or staff manual), which could leave you open to disciplinary action from your employer.

Your tax situation will become more complex and you should seek advice on this point so as not to be left with a sudden and surprising tax bill. It is very likely that you will be able to defer your Class 2 National Insurance Contributions (the ‘weekly stamp’) depending on the level of your employed income.

See also: Tax code changes when setting up as a sole trader while working

Also, be very careful about less obvious things such as car insurance, especially if you use an employer-provided car to transport yourself and the goods/tools of your new self-employed trade.