Free wins under about 30 people in a garden, paid wins the moment you need staff, a bar or a roof. Both routes have upsides.
A free space can be the smart call for small, casual gatherings. A private garden or home needs no public-space licence, and a Royal Park is cheap for tight budgets. According to
The Royal Parks, groups can meet without prior permission up to a per-park cap, with Hyde Park and Regent's Park allowing up to 40, though any alcohol must be free of charge rather than sold, and larger or commercial gatherings need a licence. The catch is capacity, plus DIY setup, cleanup, weather cover, catering and loos. At home, noise matters too.
GOV.UK notes that councils can act on statutory noise nuisance at any time of day or night.
A paid bar earns its keep on scale and ease. You get real capacity, staff, a weather-proof room and licensing already handled, so you host rather than run the logistics. If you still fancy the open-air feel without the DIY, you can browse
London venues with a patio that keep the outdoor vibe covered.