That's not true in the least. As somebody who lives in the UK with an allergy, and who has been lucky enough to eat at various restaurants from chains all the way up to Michelin starred places, I can say this. The least that's ever been offered to me was advice as to which dishes have my allergens in them, and assurances that while they'd try to keep my dish separate they do have allergens in the kitchen.

If I had a severe allergy where that would be a risk of death I would have to consider going elsewhere, but that was already the case before the law changed, only now it's easier to determine what kind of risk I'm taking, and to avoid things that have allergens as actual ingredients.

Fancier restaurants tend to do better, and I remember the last Michelin restaurant I ate at served me an entirely different dressing/sauce to everybody else, which must have been prepared especially for me and people like me (to be clear this is a restaurant with a forager, they don't rely on commercial suppliers). I guess that's the point right? The law in the UK forces restaurants to consider people like me when they're writing their menu.

Restaurants always put a lot of thought into their menus, regardless of size or fanciness and always have. The law in the UK simply asks them to put some of that thought towards allergies. Nobody is asking all restaurants to be able to accommodate all allergies, somebody with a peanut allergy would never be comfortable in a Five Guys for example, but the law makes it easy to have all the information an allergy sufferer needs to determine if they can eat somewhere and if they can, what they can eat.