HTC has announced that UK owners of the One M8 will begin seeing the Marshmallow update hit their phones over the next 48 hours. The company recently confirmed that customers in the U.S., Canada and other locations would also begin seeing the update roll out. HTC's UK Twitter account confirmed the update with the following statement:

Following the One M8's push, HTC will begin pushing the update out for the One M9 in the UK as well. If you own either of these smartphones, and reside in the UK, be sure to keep an eye out for the update to hit your phone.

Source: HTC UK (Twitter)