It's easier to actually do it in person though, even with the fuel price and driving. I'm never a big fan of posting my passport away, Special Delivery or not!
If I lived in London I'd definitely try and head to the embassy there.
Tourist visa from home is certainly the easiest either way. I'm actually in the UK right now and heading back in a couple of weeks for only 35 days, but I'm thinking of getting the visa first, given the Horror stories that are going around I feel a full visa is safer. And I'll appreciate it when I don't have to go and extend 1 morning, all for the sake of 5 extra days!!
Remember, I said there's not alot in it, I didn't suggest it was the all round best thing to do. It is always advisable to get a proper visa if you have the time though. It's easier, and it makes your entry into the country more likely. But, not necessarily cheaper.
My bike I already owned for years, or I could always borrow a friends bike. The fuel cost is maybe 10 baht. failing that they have a free shuttle bus service to and from where the immigration office is in Chiang Mai, runs all day. 10 photos costs 100 baht (£2) appose to two visa photos in the UK which would cost £5. Photo copies cost 1 baht each. So 2 copies, one of the photo page, one of the page with stamp and the departure card together, total = 2 baht (5 pennies).