What is the best visa option for entering Thailand to marry my Thai girlfriend?

All you can do is make sure you have the 3 proof ofz

proof of funds (20K baht or the equivalent) in CASH

proof of onward travel (within 30-60 days)

proof of lodging

Have those, hope for the best AND if you're denied entry by air, see if your airline will let you fly to a neighboring country and then enter thailand by land. You won't have any issues entering by land. It's air entries that get the scrutiny.

You apply for ALL thai visas from the thai consulate in London online now. Here's the link;

The real question IS can you apply online at the thai consulate in London and get everything submitted and approved in time for you to fly here in two weeks. There is no walk up service at the thai consulate in london. it's online.

Seems like you're starting this process awfully close to when you're traveling here :/

IF you don't have the time to get a single entry tourist visa you can fly in and get a 30 day visa exempt entry AND then get a 30 day extension on that for 1900baht at the immigration office once you're here.

You can come as a normal tourist then extend,

No one can tell you what Immigration will do when you get here,

Try to get SETV in your home country, Immigration officer may or may not let you enter Thailand. Maybe try to flying into smaller airports. Or flying to neighbor country and enter Thailand on land could be an option.

Just s useless though.. Maybe bring all possible papers along with 3-proof-of-s. Don't think any other papers could help much, though. Maybe it helps for IO to be a bit of leniency one of times if, for example, you have your own wedding invitation and related papers. Anyway if there are some papers, why not bring with you just in case..