plain formとき tense

When…

MNN23-2

When the present tense is attached to とき, it indicates that the action has not been completed,
when past tense is attached, it signifies the action has been completed.

イギリスへ 行く とき、ロバートさんに 会いました。
I saw Robert when I went (while I was going) to the UK.
イギリスへ 行った とき、ロバートさんに 会いました。
I saw Robert when I went to the UK (after I arrived at UK).