I know that I am old and probably out of touch but I cannot for the life of me see how any meal could warrant a £100 per head irrespective of who the chef is and what food he/she has cooked
The wine would make up a hefty percentage of this.
Depends on the establishment. Most high end (say, just for example, anything with one or more stars) places will have a tasting menu where food alone will be £90 - £140 pp. Of course, there are only about 175 restaurants in the UK with one or more stars, so it's a tiny % of the catering induestry where you'll pay £100 a head just for the food - but they definitely exist.
I think The Araki currently has the most expensive set menu - £310.