Can I get an e-tourist visa and extend it for marriage to a Thai national?

There's far more people who do the work on this group than I do (y)

Get a single entry tourist visa from the thai consulate in London, using the online eVisa system, come here, get stamped in for 60 days, which you can extend for 30 more days for 1900baht once you're here.

Now whether you can get your act together, get the documents needed to get married from your embassy, get them translated, get them certified at the MFA, then get married in that amount of time is totally up to you 🙂

That's your best option that gives you the longest time so you can get everything sorted out, get the marriage registered and then start down the path to a "marriage visa"

Good luck with it