Licensing depends on who is supplying what and where. In England and Wales, selling alcohol or putting on regulated entertainment such as live or recorded music and dancing needs proper authorisation.
If your venue already holds a premises licence, the hard part is done. According to
GOV.UK, any business supplying alcohol on a permanent basis needs a premises licence. That's why licensed spaces such as the city's
cocktail party venues in London take the admin off your plate, though you should still confirm your plans fit the licence's activities and hours.
If you're using an unlicensed space such as a hall, a garden or a park, you'll likely need a Temporary Event Notice for a one-off celebration.
GOV.UK sets out the rules: fewer than 500 people, a £21 fee, and an application at least 10 clear working days before the event. Free drinks at a closed private party may fall outside licensing, but the line is fine, so check with your local council first.
This all applies in England and Wales; Scotland and Northern Ireland run separate regimes.
Pro tip: Include your drinks arrangements, entertainment and finishing time in the enquiry. The host can then confirm which activities the venue’s permissions cover.