The key is to establish where the “place of supply” is. HMRC’s notice 741A covers this: https://www.gov.uk/guidance/vat-place-of-supply-of-services-notice-741a#sec9
From your description, sections 9.3 and 9.4 may apply. If this is the case, the place of supply is the foreign country where the conference is being held. UK VAT wouldn’t be chargeable to either UK or overseas attendees – but you may have some liability for VAT or similar taxes in the foreign country.
My only doubts on this is that your description does appear to include a wider bundle of services than HMRC’s wording – would they all count as “ancillary”? If the service package you’re supplying doesn’t fit within the HMRC definitions in 9.3/9.4, you’re back to the general rule of charging UK VAT to UK-based customers.