SE measures 1 inch below the crotch seam and Giles measures across the entire thigh at the crotch seam (you need to look at pics to understand). That's why they are different. Sounds like you are good to go.
SE size chart suggests otherwise (the thigh of 34 is 12.2 rather than 12.5). My thigh now is around 12.8
Yeah, I just realized they are a bit different. Well I've only just seen how IH measure…
Thank you, kit99bar