Frankly, BT preparing itself for a sale is the only thing that does make sense right now. There had to be something behind such a huge layoff number.
The company is rife with the rumour that management is trying to dress the company up for some kind of transaction, possibly involving Deutsche Telekom.
http://www.businessinsider.com/savage-layoffs-at-bt-could-be-preparing-the-company-for-a-sale-2018-5