Could you get away with comfortably riding an 18" Ragley Ti if you're 5'7" with a 30" inside leg?
Comfortably? Nah. I'm that height/inseam and had a 16". The 18" was just too long and high
Here's a photo of mine set up
[url= https://farm6.staticflickr.com/5140/5465246522_a96ef8f91d_b.jp g" target="_blank">https://farm6.staticflickr.com/5140/5465246522_a96ef8f91d_b.jp g"/> [/img][/url][url= https://flic.kr/p/9jWPoj ]P1010623[/url] by [url= https://www.flickr.com/photos/druidh2000/ ]Colin Cadden[/url], on Flickr
I'm 5'10 and 32 inch inside leg, had (til today!) a medium. I'd say the overall sizing could work for you but it has a fairly high top tube and it's generally fairly tall, which'll most likely encroach. (I wouldn't say too long necessarily- it was a fairly long bike when it was new but typical wheelbases and reach have increased a lot since then)
Mine looked like this:
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So you'd be OK for seatpost etc... I'd recommend the bike to the hilt, it's a honest to god ****ing masterpiece tbh but I do reckon you'd be better on a small. I feel like I might have gone mad selling mine tbh but it's off to a good home.
Remember the geometry (NOT the frame shape, but the geo) is identical to the original Blue Pig and Mmmbop and there's a lot more of those around to try.