If I change the Dx code for Ektar 100 to 800 ISO. Take pictures at night/ cloudy day and then develop it at the High Street studio (without letting them know about ISO), will I get good results?
You'll get massively underexposed negatives with very little density. At normal settings, your prints or positive scans from such negatives will be very dark.
Your photo processor will probably attempt to compensate somewhat when printing from the negatives or when scanning them, but the results will be noticeably worse than if you had informed them you underexposed by three stops so that they could have "pushed" the development of the film. Once film has been developed, it can't be "reprocessed" the way raw digital files can.
For fuller discussions of the effect of shooting film at other than its rated "box" speed, please see the following questions and their answers here at Photography SE: