There is no legal requirement for you to have IFS into the UK/EU, although if you're doing a lot of business in the UK, it is increasingly an expectation/mandatory requirement for some of the bigger potential customers here - there is a drive by the big retailers to have certification in place throughout every step of the supply chain. It's worth noting that if it is the UK market in which you're primarily interested, the preference would be for BRC Agents & Brokers rather than the IFS Broker standard. Looking at the current GFSI benchmark recognition (https://mygfsi.com/h...nt/recognition/) it appears that the BRC version is recognised, but IFS isn't.
I guess the summary is: No it's not mandatory, but it may add value an either help retain customers, or open the door to new ones. I could be biased (I put BRC Agents & Brokers in during the first year it was available), but it seems to have been very well received by the UK market.
The "audit" will take place at wherever you are physically based, rather than the location in which your transactions/storage etc are happening. As these standards are for agents/brokers, they're not expecting to see an actual manufacturing/storage site - they won't do a physical inspection of the premises, but would instead review your food safety systems, records, do a traceability exercise or two etc, none of which require an actual manufacturing premises.
N.B. This may potentially be done as a remote audit, rather than sending an auditor to your location - I'm not sure of the approach for first certification audits now, as there has been some change due to the pandemic this year.
I'd still very much recommend talking to some potential certification bodies - they'll be able to confirm exactly what sort of audit they think would be required, how much it will cost, how they would like to do it etc.