@Garry **********
, you can't ever renew a visa or an extension of stay you get inside the country. They run out when they expire and you buy a new one.
If you are going to get another one from the consulate in london it's the exact same process.
IF you are going to apply for a yearly extension of stay at the immigration office you just meet the financial requirements of 65K baht a month in income from a notarized letter from your consulate or 800K baht in a thai bank account in your name for 60 days prior to your application for a yearly extension.