One could say if you are a good enough magician you should not need to ask for tips.

only joking.
Personally I don't think you should ask for tips.
For starters they probably haven't asked to see you perform, and would be more than happy to just eat the meal that they came out to the restaurant for in the first place. Without any interuptions other than being asked if they would like more drinks. But, being british and all we would never be so rude as to decline the offer of some light entertainment between courses and watch tentatively as the likes of us perform our prestidigitation to small applause.
My opinion - "If you gotta ask for TIPS, you ain't charging enough!"
You need to make the restaurant manager believe that you are a main attraction for his buisness.