I didnt think UK had any class A anymore. Makes me wonder how old your maps actually are.
You are correct i your assessment. I would point out that the vice versa if you will woth danger areas, if the height is designated as al altitude (eg 5000'), you must be 5001' or above to be clear of danger area. If it is a flight level, (egFL75), then you are clear by being at FL75. This would be the same in your CTA or airway example. Base of an airway is FL75, then you can cross the airway perpendicular at FL75. Hope this help and i await to be corrected.