British Nationals require a visa to visit Laos.
Visas are available on arrival at border crossings by air/boat/road.
The visa is a single entry (30 day) tourist visa, costing 35 USD.
Application in person
For more information you can contact the Laotian Embassy in London on 020 7402 3770 or by email laosemblondon@gmail.com
Detailed information can also be found on here: https://www.worldtravelguide.net/guides/asia/laos/passport-visa/