The following assumes that you hold a British passport:
Singapore: "You do not normally need visas to enter Singapore for up to 30 days stay for tourism, business discussions or social visits".
http://tinyurl.com/24w6x8h
Vietnam: "A visa is required for Vietnam. Ensure that you obtain the correct visa for the purpose and destination of your travel, via your closest Vietnamese Embassy"
http://tinyurl.com/2aaarvb
Cambodia: "Visas for Cambodia can be obtained on arrival. Two passport photographs are required. A tourist visa costs US $20 for one month and can be extended for only one extra month. The Cambodian Ministry of Foreign Affairs operates an electronic visa ("e-Visa") facility for tourist visas only. The e-Visa costs US $20. e-Visa can only be used at the main entry crossings with the immigration IT system. Applications should be made through the Cambodian Ministry of Foreign Affairs' website, where full terms and conditions are also listed, with information on which border entry points accept the e-Visa."
http://tinyurl.com/23l3lns
Laos: "Visa conditions change regularly. For the most up to date information contact the nearest Laos Embassy or Consulate"
http://tinyurl.com/28nt74s
and "There is no Laos Embassy in London or UK. The nearest embassy to the UK is in Paris, France. Visas, valid for 14 days and costing US$30, can be obtained on arrival at Vientiane or Luang