Any specific Thai Embassy/Thai Consulate has discretion to set their Consular Service Fees and Rates, just like they have discretion for additional requirements and request additional documents, for example, specifically relating to DTV E–Visa, a Thai Embassy/Thai Consulate may say seasoning of the minimum 500,000.- Thai Baht in your bank account for a minimum of 1 month, 3 months, even 6 months, or, another example is that 500k Thai Baht must be a personal account (savings or checking) at a retail, brick & mortar bank vs. a brokerage account or Wise/Revolut account or investment account, etc. etc.
Specifically for Thai E–Visa Fees, the most egregious example is the Thai Embassy in Wellington where all their Consular Fees, Rates, and Charges are well above their neighbors (Thai Embassy in Canberra or Thai Consulate in Sydney)
inexplicably expensive at the Thai Embassy in Wellington
However, the only locations where the Thai E–Visa Fees are exactly as stated in the regulations as published in the Thai Royal Gazette are at: Thai Embassy in Vientiane and Thai Consulate in Savannakhet as they are they only locations that accept Thai Baht
if your intentions are to move to and live in Thailand, or, at least spend more time in Thailand than in the UK, then it's just easier to get a Thai Marriage Certificate from Thailand (get married in Thailand) than having an UK Marriage Certificate (getting married in the UK) and getting any UK Marriage Certificate legalised at Milton Keynes, then at the Thai Embassy in London, then at the Department of Consular Affairs in Bangkok, perhaps even also at the British Embassy in Bangkok....
* All applicants must be residing within the UK/Ireland/UK territories when apply for the Visa. The Royal Thai Embassy has the right to cancel the visa applications should you fail to provide the evidence of your presence within the UK/Ireland/UK territories while applying for the visa.
if it is any of these 93 places in the world, then you don't need a Thai E–Visa beforehand and get a free Visa Exempt Stamp by the Thai Immigration Officer for a Stay of 60 Days