We've already put together a guide of deals for iPhone, Apple Watch, iPad, MacBook, and other Apple products, as featured in this year's Black Friday ads, which you should check it out while you wait for Apple to unveil its own deals.
The study doesn't explain which products it checked, beyond stating "popular products in TVs, cameras and fitness trackers", so the 87% figure might not apply to cheaper or more niche products that consumers might pick up.
In recent years, several stores have begun extending Black Friday into Thanksgiving Day itself, opening after the turkey has been finished to allow maximum potential shopping time.
Alex Neill, managing director of home products and services at Which?, said: "The results of our investigation will disappoint many who are expecting nothing but bargains this Black Friday".
When is the best time to buy? Apple is advising users to mark their calendars for the upcoming four-day shopping event that starts this Friday.
Which? analysed deals at leading retailers Currys PC World, Amazon, John Lewis and Argos. It was cheaper on at least 60 occasions afterwards, starting with a further £20 discount in mid December and dropping to £39.99 for at least 21 days during March and April 2018.
An Amazon spokesperson said: "We offer many thousands of incredible deals around Black Friday representing millions of pounds of savings for our customers across a vast selection of products". Target, Best Buy and Walmart are among the retailers offering some of the hottest Black Friday deals for this year's shopping event, which takes place November 23.
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John Lewis said: "Prices are matched throughout the year as part of its Never Knowingly Undersold commitment, meaning that when competitors run promotions, prices are lowered to match, and that the Black Friday period is no different to that".
Thanksgiving is just around the corner, and along with tables full of food, that also means the arrival of the shopping holiday known as Black Friday.
Currys PC World customers were told they could save £10 on a Sony compact camera advertised as £189 on Black Friday.
"The LG 60UJ630V, a 60" 4K TV, was advertised at £799 by Amazon on Black Friday but this wasn't its cheapest price.
We're not aiming to flood you with deals - we're looking through all the major retailer offerings to see when the tech we love (or like the look of) gets a meaty discount - and point out when some items shouldn't be seen as great value.
Sign up to your favourite retailers' newsletters for inside tips, or check their social media pages ahead of the sales for any clues or hints to what will be on offer.