I haven't stayed in the Premier Inn in Sheffield, but I've stayed at quite a few PI's stretching from downtown London and Heathrow up to Dundee, Scotland. They've all been fine. The large beds are very comfortable. If the hotel offers a twin-bedded room and you book that, they'll make up the day bed (not a pull-out sofa) as a single bed, and I've found those perfectly OK, too, just not as extra-comfortable as the large beds.
I've read complaints about slow Wi-Fi if you don't upgrade, but I haven't noticed problems with run-of-the-mill web browsing. I guess you'd need to upgrade to stream video.
I've never eaten at Premier Inn. Opinions about the breakfast seem pretty divided, with some folks saying the hot breakfast is a good deal and others calling it mediocre. I think the PIs have a "children eat breakfast free" deal, which could be driving some of the positive comments.
The physical plant at every PI I've been to has been great quality for the price--they're an especially good deal in the summer when you want to book a place with a/c, but they save money on customer-service staffing. It's very difficult to get someone on the phone, for example, though when I've communicated with someone in person, they've always been pleasant.
There's a barbecue place in Sheffield that Americans should probably avoid; its barbecue is way too sweet. I've checked a map, and I'm pretty sure the place is named Smoke BBQ.