As tempting as it seems to wear tennis shoes with your tux, don't do it. I think it looks ridiculous. If you're 14 years old, maybe give it a shot. In general, don't portray anything that says 'I'm too cool and I don't care.'
I'm not sure it pays to do anything remotely public in Britain. It's such a spiteful society. People seem to enjoy making your life hard for the sake of it.
That some Forms of Government are preferable to others, cannot be doubted; yet neither our Saviour, nor his Apostles have decided where that Preference is due.

