Im 20 male british citizen and am going to thailand 10th of june on 60 day tourist visa, what I want to know is when should I apply and should I visit the embassy in london or apply by mail through hull.

A single entry tourist visa is very easy to obtain. Apply wherever is most convenient to you. Be aware that London only does personal applications, whereas the Consulates in the UK will permit postal applications. You need to enter Thailand within 3 months of the date of issue of the visa.

Thx, also do I need an appointment for the london embassy

Don't believe any Embassy/Consulate requires an appointment but they normally only accept applications in the morning, usually 9 to noon.

Send your application to Hull or Cardiff... very easy by post

Good Luck