You really think they'd just erase the photos of UK PMs? Please. If there was some kind of conspiracy there, don't you think hiding the information too (vs linking directly to the pages that have it) would be a more effective strategy?
I know it's fun to jump on Google as the dictator erasing history here, but have you really thought this position through? Especially when one of Google's big industries is education, and kids are going to write papers about WW2, and various British PMs?
The most likely explanation is that a google employee added the photo on a block list in support of BLM.
It also seems clear that there isn't one global list of these people and their portraits Google uses to feed these boxes: When I compare the screenshot someone posted below with what I see, many of the leaders have different portraits.