it's not called Visa on Arrival, that's something completely different that you have to pay for. Coming from UK it's a visa exemption. Common mistake though.
You get visa exemption of 30 days when you arrive in Thailand. Unless you are staying longer than 30 days each time, no visa required. Visa exempt can be extended by 30 days at local Thail immigration office.
Go to Krabi immigration, but not open until 18th. Or risk overstay 500 bt a day at the airport. Immigration office should not charge for overstay due to closure. However if already done 1 30 day extension then you cannot get another. Immigration can be asked for 7 day " must leave" stamp.