I would not allow the dealer to do it - as others have said get a decent detailer to do it. The actual stuff is quite cheap.
For info I bought a ceramic coating kit from china ( very cheap )and have used it to great effect to give a "gloss" finish to a set of Mini alloy wheels that were a less than shiny satin finish from new. Results are good and seem to be standing the test of time. I had a bit dribble out of the bottle into the box it came in and the resulting pool of hardened material is indeed very hard and won't wash off.
Stuff goes on and needs to stay dry for a good few hours to be effective.
Is it worth £700? Only if its applied properly. Get it applied better and for less by using a decent detailer. Is it needed? - that depends on how you are prepared to care for the car - decent 2 bucket wash and rinses, done often, with decent products would work just as well. Can't do much harm either way.
