The relevant legislation makes no reference to 'fire marshals', per se. Therefore even if your workplace had several thousand employees, there wouldn't be any specific need for any of those employees to be officially designated as 'fire marshals'.
All that the legislation states is that, where there are 5 or more employees "the responsible person must make and give effect to such arrangements as are appropriate, having regard to the size of his undertaking and the nature of its activities, for the effective planning, organisation, control, monitoring and review of the preventive and protective measures" and that he must "record the arrangements". (The 'responsible person is the employer or the person having control over the building).
The 'responsible person' is also required to ensure that employees are provided with adequate training in fire safety but, again, that doesn't require anyone to be appointed as a fire marshal.
https://www.legislation.gov.uk/uksi/2005/1541/contents/made
When I worked on a busy railway station, there was an automatic assumption that the duty supervisor would take on the role of fire marshal in the event of a fire occurring. (If he/she could actually find it, he/she was meant to don a blue hi-vis vest with 'Fire Marshal' printed on it). However that was simply stating that the person in charge of the station would take the lead, which was self-evident anyway. (All platform staff, together with all train crew, were periodically given practical training in fire-fighting by Norfolk Fire & Rescue Service, where we actually had to bring some large blazes under control on each occasion).
Whoever's the boss (or 'duty boss') where you work can take on the role of fire marshal, without any need for a formal designation as such, as long as he/she is fully conversant with evacuation procedures (and preferably has been given practical training in the use of fire extinguishers)