What are the visa rules for traveling in Southeast Asia as a UK passport holder?

Hi, Been doing some research on Visas and wanted to know if the notes I've got are correct Before i Departure on my trip around some of SEA. Planning to go for 80-90 days in total, However i am not sure how long i want to stay in the countries i would be visiting. So i have a UK passport, I'm Flying into Thailand which gives me a 30 day free Visa Entry pass, if it needed to be extended i could do a border run or get a 30 day stamp for 1900baht from an immigration office. Going into Cambodia at the border i can buy a 30 day tourist visa for $30 or a E-visa that allows an extension for another 30 days for $45. Going into Vietnam from border i get a 15 day free visa stamp or can get an E-visa before entering for 30 days for $25 and extending it if needed with a border run. and entering Laos by border i can buy a 30 day tourist visa for $35 and if it needed to be extended i can pay per day from travel agencies or do a border run. Thanks.