這是關鍵 如果您留在Airside 在同一機場不同航站樓內過境不會進入英國 At landside你就進入了英國

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.

你離開 LGW 到 LHR 意味著你必須進入英國,  you definitely need Transit Visa.