What is the website Mathilda is viewing in the first episode of Humans?

It is most likely non-existing.

In my experience websites used in movies or TV shows are non-existent, or if they exist they're "fake", i.e. specifically developed to target the viewers, as some sort of promotional tool. This happens for various reasons:

Some TV shows make up websites to skirt copyright issues. Others use fictional sites as plot devices.

The main problem with using an existing website is that its owners have to give permission; the same is true for brands of food, cars, etc. so writers are instructed to avoid existing brands.