Welcome steve, From memory i believe this is 750mm this may be smaller if used by less than 5 persons, & 900mm for wheel chair access, i stand corrected. however there may be factors that you may will need to quantify in your risk assessment, eg 1 or 1000 persons expected to use these routes. The following link provides good information: https://www.gov.uk/searc...;tab=government-results. down load the specific guidance eg; office, small place of assembly and look at the section on escape routes.

Perhaps a little easier to navigate is this part of the gov.uk website which provides links to the fire safety guidance literature provided by HMG Choose the guide appropriate for your building's use & download for free