I could not find the report of the study by Syzygy and marketing intelligence firm Attest, although I wouldn't be surprised if the report has still been kept confidential by whoever commissioned the study. Market information that provides competitive benefits is part of the point of commissioning such studies (Forbes).

Surveys and polls reporting the number of people who delete Facebook accounts are scarce. To the best of my knowledge, no other poll has been conducted on this topic in the UK. Additionally, Gallup has not yet conducted a poll on this topic. The Pew Research Center has conducted a general survey on Facebook and the American population (emphasis added in the excerpt). Unfortunately, none of the survey questions had to do with permanently deleting a Facebook account.

Most notably, 44% of younger users (those ages 18 to 29) say they have deleted the Facebook app from their phone in the past year, nearly four times the share of users ages 65 and older (12%) who have done so.

Of course, deleting the app is different from deleting a profile. With such a high number, I wouldn't be surprised if 5% of Britons deleted their Facebook profile. While the rest of the Pew article is interesting, it doesn't directly relate to the question. I encourage you to read it if the subject is fascinating.

I did find two other relevant surveys. cnet reported that (emphasis added)

A new survey from the app "Blind' found that up to 31 percent of tech workers are planning to delete Facebook.

(Also, there's a big difference between saying you're going to delete Facebook and actually deleting Facebook.)

The other relevant survey was conducted by Tech.pinions (emphasis added).

We ran a study across 1000 Americans who are representative of the US population in gender and age.

Seventeen percent deleted their Facebook app from their phone, 11% deleted from other devices, and 9% deleted their [Facebook] account altogether.

Business Insider, Fortune, and techradar have reported on this statistic.

It is interesting to note that if the campaignlive and tech.pinions statistics are accurate, 5% of Britons and 10% of Americans have left Facebook.

Finally, there is a survey conducted by The Atlantic that @Fizz identified (emphasis added).

This survey is not scientific—The Atlantic pushed it out to readers on Facebook and Twitter, in newsletters, and through our membership program, no doubt skewing the demographics of the sample. The 2,218 respondents were 82 percent white and largely based in the United States. Fifty-nine percent were female, 40 percent were male, and 1 percent were nonbinary. A variety of ages are represented, though on the whole the sample leans older.

A small but not insignificant portion of people—9.6 percent—said they deleted or deactivated their Facebook account as a result of the news.

This 9.6% roughly agrees with the 9% from Tech.pinions survey of representative Americans.

Is this WPP study corroborated by other sources on the outflux?

Yes, the results of the WPP study are corroborated by other sources and don't appear to be unlikely. Of course, it would be better if Gallup or Pew conducts a poll/survey in the UK. Until then, this will be the best answer that can be provided.

This answer will be updated as new information arises. I will incorporate helpful comments into the answer.