BT is a multi billion dollar company, not to mention that sharesave isn't the same as an equity compensation.
Share As You Earn SAYE is a savings plan in the UK which allows employees to save money from their salary in company shares.
The UK has really weird schemes because people have historically had no pension or savings plans from their employees (most PAYE workers still do not have pension as the date mandated by law is always being deferred).
With SAYE as far as i know the employer is not allowed to grant you equity, what they can do is give a fixed yearly rate (usually heavily discounted) for share purchases, but it's not as sweet as it seems.
The dividends and the equity rights from the shares belong to the employer not the employee, this is basically a way to allow employers issue shares (in large volumes) without losing control over the company, having to do payouts, and decreasing the market value of their normal shares as SAYE shares are not tradeable.
It also allows employers to bypass various laws preventing normal employees from having too high of a share of the company, and ties employees to their employer since not only do they rely on it for their salary but also as their investment/savings provider and since SAYE plans are either 5 or 3 years long it pretty much means that invested employees will not living the company during the SAYE period unless they want to lose their investment (and yes they will lose it).
BT's Sharesave is also a "unicorn" and from the current buy-in value it will probably won't repeat it self, yes a few people who saved up the max amount (225 GBP a month) gotten about 80K in return. But and this is a big but those were the 1st shares issues at 80p per share, when they matured the shares closed at over 300pp/s the last round of the SAYE program had a buyin of 250pp/s so pretty much no one will see these returns again.
P.S.
The money you gain for SAYE Isn't tax free you pay capital gains tax on it if you sell them once they are matured.
Also since SAYE with all of it's bells and whistles is a company options plan (with heavy tax incentives to the employer) it's still a risk, some people got huge returns others didn't since the share price was lower than the option price.