Referral link: 1 month free referral
What you get: a free month (plus a free sim if you’re not a Smarty customer yet) of any plan.
Name of company: Smarty
What they do: Cheapest mobile operator in UK
What I get: a free month.
I just tested it (this week) with a friend, and can confirm this link works as of 30/4/2020, and will work thereafter.
Some competitive plans you can choose: 1 month unlimited text/calls+data (20gbp) or +30gb data (10gbp). There are others.
Been a customer for one month and they are good. They use Three’s network.