No. Only you can judge whether or not to tell your employer.
The Volunteers Employment Protection Act 1973 makes it an offence for an employer to discriminate against, disadvantage or dismiss an employee or prospective employee for rendering service in the Reserve Forces. Also, no one from the Directorate of Reserve Forces and Youth Development or your unit will contact your employer on your behalf without your prior consent.
To help us provide the support you require we encourage all Reserve Forces personnel to discuss their military service with their employer.
Your employer may be more willing to release you for civil defence or other emergency assistance if they are aware of your Reserve Forces involvement in advance.