What visa should I apply for in Hong Kong to later change to a marriage Non-O visa in Thailand?

Hi All, Some advise required please. I have a british passport and been working here in Thailand on and off for 2 years and been on a non B with work permit. In between times I have retuned to the UK applied for a METV, my latest visa expires shortly and I have just applied for the 30 day extension taking me to the 12th May. I will getting married on the 7th April however I can not get an appointment at the Embassy for the Affirmation of freedom to marry until the 13th May (just my luck). I therefore intend to fly to HK the first week of May. Can i ask what visa you would suggest I apply for in HK? in order to then change to the marriage Non O visa here? Or do you think it is possible to return to Thailand on the 30 day visa exempt and apply for the marriage Non O visa once i return?