Yes, just like the food in Italian restaurants here is completely different from the food you'd eat in an itlaian family!
They just modify the recipes to appeal to the british palate, which is more keen on fatty savoury dishes generally.
I would never go to a Greek restaurant, I just can't stand the concepts or the menus and the bloody balalalaikas. Same with IKEA and horrible Swedish muck, all chewy rollmop herrings and oatmeal biscuits, same with Thai all watery over-priced rubbish, the only really British dish is a Vindaloo. Foreigners eat foreign food and that is their own fault, we can rise above it.
Proper greek food is lovely, lovely fresh salads and proper kebabs.
I have greek friends so maybe have tried some more authentic stuff.
Saying that I lived with 3 italian guys for a while and when they cooked it was amazing and nothing like the stodgy stuff you get in italian restaurants over here. Same when I went to Italy, very different.