这是关键 如果您留在Airside 在同一机场不同航站楼内过境不会进入英国 At landside你就进入了英国

Airside - behind security check point, i.e. the area leads to gates.

Landside - before security check point, or exit the gate area to the arrival hall.

你离开 LGW 到 LHR 意味着你必须进入英国,  you definitely need Transit Visa.