Well, I think that everyone is piling on you here OP and being a bit harsh.
The vibe that I'm getting is that you don't wanna spend your precious free time at a mediocre chain restaurant, but you can't voice that because it's the boss' "treat" and so youre gonna have to suck it up and go along with it. I totally get that.
I feel the same about chain restaurants. I've never been to Las Iguanas and I likely never will, certainly not for a social evening. Thats my personal stance on chain restaurants.
I'm not posh but I spent 5 years working in restaurants as a waitress, and I have qualifications in wine etc. And to me there is just a totally clear difference between chain restaurants and independents. They are not comparable.
Depending on the vibe of the workplace, I'd go as far as to say that it's not really a suitable choice. Although bear in mind I've never been.
Now that I have read this thread, maybe I will go to Las Iguanas to feel a bit more 'current'. But only on my own, maybe while I'm waiting to go on to meet friends for a drink. Or if I get hungry whilst out shopping - that's what chain restaurants are for, if you ask me. A quick pit stop.