I've been with Smarty for about a year, £12pm for 100gb plus unlimited minutes and text, and 5G. I've not had any problems with them, porting my old number took about a day and a half. The app is really good. I also got a free month for recommending a friend, they got a free month too.
Customer service is excellent.
They offered me unlimited data / min and txt for £18pm. And there's no long contracts, just pay monthly, cancel when you like.
By-the-way, use my referral link if you fancy a £10 gift card, we'll both get a choice of a Gift Card from Amazon.co.uk, John Lewis Partnership or Uber..
https://i.smarty.co.uk/UZbvkkx