Andy
If you are being asked to give the advice because the shopkeeper knows you work for the fire service then this brings some liability on the fire authority and you should perhaps refer them to the fire safety department.
Its up to the shopkeeper to know the basics of fire safety law and not to lock exit doors and not something any individual firefighter is in a position to change.
Its also up to the fire authority- not individual firefighters to worry about how they will enforce fire safety and make sure shops in their area will not lock fire exit doors.
If the shopkeeper is asking you because he knows you work for the fire service and will on the side give him some advice on the cheap as a private individual then thats a private and personal decision and entirely up to you.
Yes of course like in any field you can undercut professional consultants because you will not have insurance, accountants, staff, offices, CPD, training, corporation tax, VAT and 1001 other business overheads to find. And people always want a bargain and many are prepared to turn to the grey economy in all walks of life to do this.
The way it was supposed to work is that the guidance documents should contain sufficient information for the responsible person to carry out their own risk assessment in smaller premises. If they need advice they can ask the fire service who will be pleased to help with specific queries. If they still are unsure they should consider seeking assistance from a competent person, either from amongst their staff, from within their professional or trade organisation or a fire consultant. Fire consultants are often happy to offer free advice too-we are not all on the make all the time!
If on the other hand you are aware of a group of local businesses struggling to understand and deal with the law, why not suggest the fire safety department put on a free seminar and open evening for them at the fire station, talk them through the basics, even produce and give out a template for a simple fire risk assessment, then go and sample the standards afterwards on a goodwill basis? As a private consultant wishing to support small business in my area this is something I am doing regularly in my area, usually free of charge, and providing suitable risk assessment templates for them to fill in. Putting one on next week for 100 holiday cottage owners - why cant the fire service do this too?