I too am quite scrawny. What I have found in London is that most tailors will only do made to measure within the same range as off-the-peg. So if your problem is that you can't find suits small (or large) enough in most shops, then going made-to-measure probably won't help. You may have to go bespoke, but it will definitely be more than £300. I have found some good Ted Baker suits in smaller sizes at Slater's for about £250. Branches are outside the city centre, but you can give them a call and ask what they have. Be sure to specify "Ralph's Room" which their more upmarket selection.As an aside, some tailors will offer to butcher an off-the-peg suit in a near size to make it "made to measure", but it's really not worth it. From experience, even if the result doesn't look terrible, the fabric will pull back into shape and tear out the new stitching over the course of a couple of years.