The impact of McDonald’s UK expansion on existing chains was interesting. There tends (to this day) to be a sizeable independent fast food market (Fish and Chip shops, Indian/Chinese takeaway places). Chain wise, prior to the arrival of McDonalds, there was basically Wimpy (in the UK from the 50s) and Kentucky Fried Chicken (before it abbreviated to KFC). KFC pre-dated McDonalds (1974), Pizza Hut (about 1980) and Burger King(1977) in the UK by a decade (First branch was 1965 - surprisingly in Preston, Lancashire - the first London site was in ‘68).
Without McDonalds, I think Wimpy would probably have benefitted most in the UK - as a straight burger joint, they nearly got squeezed out by McDonalds. The rapid spread of the Golden Arches was only matched by the rate of their decline. The table service approach was their downfall - McDonalds was seen as fast and modern. However, by the 80s, I think if they hadn’t adapted, then Burger King would have had the same impact anyway.