No, your employer cannot lawfully force you to use facilities that do not align with your gender identity. Under the Equality Act 2010, employers have a duty to prevent discrimination and harassment in the workplace, and this extends to access to toilets and changing facilities.
Making a trans woman use male facilities, or a trans man use female facilities, constitutes direct discrimination and potentially harassment under employment law. Research shows that access to appropriate facilities is fundamental to workplace dignity and equality. Employers are legally required to make reasonable adjustments to ensure trans employees can use appropriate facilities without distress or discrimination.
Evidence from employment tribunal cases demonstrates that reasonable adjustments might include providing access to gender-appropriate facilities or creating additional neutral options where feasible. Guidelines from equality organisations emphasise that employers should work collaboratively with trans employees to find practical solutions that respect everyone's needs and dignity.
If your employer refuses to address facility access appropriately, you may have grounds for an employment tribunal claim for discrimination. Many people find it helpful to initially approach this through HR or union representatives, documenting any refusal to make reasonable adjustments, as this strengthens any potential legal case whilst hopefully resolving the matter constructively.