Mostly in pubs, but I did try a few places. Carvery buffet, a few different full English breakfast places. Those are the things I'd chalk up to bland but not bad. Brits truly do use less seasoning from what I could tell. Even the takeaway I tried was pretty boring, and all you have to do is fry and salt that stuff.

I don't think your comparison of fast food vs. fine dining is fair. In the US, and the few other countries I've been, "pub food" or family style restaurants are usually always good. They're not high quality but still tasty. I've only been to 7 countries so I'm not super well traveled, but the UK is the only place I've been where I consistently didn't enjoy the food. I can only remember one meal in Serbia I didn't enjoy.