What’s the easiest really depends on you, I think it’s easier to apply for the e visa online and after a few days receive the visa in pdf and print then no hassle
You could find it easier to go as you are and pay an agent 4000-5000thb to do the extension for you at immigration but risk questions at the airport when you checkin about how you will exit the country within your 30 days, buying a $12 temporary exit ticket will end the questions.
Or you could pay an agent to do the visa application while you’re in the uk and hand over all your personal info to someone
Or last but not least just turn up and over stay 8 days and pay the 3500b fine at the airport on your departure, not really recommended