Most places do average af Nachos imo. Subway (though their squeezy cheese is nice), Pizza Hut, Chaquitos (nice salsa though), Harvesters were nice but not consistent with the cheese. Benito’s Hat and Loco Mexicano were good but they're only in Central London and I only ever tried them once.
Best to go home made imo. Buy some salsa sauce and add more diced onion+tomato, bake the nachos/Doritos with Mozzarella mixed in and on top. Easy, I like to drizzle a bit of Franks Red Hot sauce on it too and add some shredded meat but that's up to the individual.