https://www.gov.uk/check-uk-visa/y/china/transit/no
官方这儿说的例外情况第一条是下面这样,之前看到有帖子说是这个情况必需过境
英国去
美国是可以24小时免签的,现在下面这意思是不需要了?
Exemptions
You don’t need a visa if you have one of the following:
a visa for Canada, New Zealand, Australia or the USA (this can be used for travel to any country)