Myactivity.google.com is a Google-owned site which exists to help you delete records of your actions on Google's sites
https://support.google.com/accounts/answer/465?co=GENIE.Platform%3DAndroid&hl=en
It's not a browser. It's simply a website like the one you're looking at now, so it can't have 'replaced' Google Chrome.
If you're finding that it's already there when you open Chrome though, tap on the number in a box at the top right of your screen, which will show all of the websites that are currently loaded in Chrome. If the number in the box is 1, that will just be the My Activity site. However if the number is large, you'll see every site that's currently loaded in Chrome, with the windows stacked on top of each other. Tap on the 'X' to close each window in turn. When you've closed the final one, Chrome will close too. Upon reopening it, you'll then see Google.co.uk back (because it's the page that Chrome defaults to when opening on an Android phone).
The foregoing assumes that you haven't mysteriously managed to change the actual Google home page by accident. (I can't see how that can be done unless you've been fiddling around within 'Settings' but stranger things have happened!). If, after you've closed all active windows in Chrome, you still find that it's opening with myactivity.google.com as its home page, do the following:
Open Chrome.
Tap the three dots at the top right.
Scroll down to, and tap on, Settings.
Under 'Advanced', tap on Homepage.
Change the line with 'myactivity.google.com' in it to show
http://www.google.co.uk/