It might be wise never to carry the Union flag on board. There's still some ancient laws about Royal Naval officers rights and duties to board you and remove it, if flown. The red duster is both the ensign and the courtesy flag for British waters.
That said, there is a growing European fashion for flying local flags. So in the UK, you might see a lot of variations on courtesy flags, even on British registered yachts in their own waters. However, see the RYA site for the real rules.
You see the European Union flag, (Blue with a ring of 12 stars), flown both as a courtesy flag and as an ensign - in this latter case with the national ensign in the corner. However, the rule is the yacht should fly the flag of its nation of registry and the EU is not a nation and you can't register a yacht as European. So the habit is wrong but I've not heard of anybody being arrested or fined for it.
So you answers:
1. Red ensign starboard spreader.
2. Canadian ensign
4. Yes - for a Canadian flagged yacht, but not for one registered in a EU state, unless it carries non-EU nationals, or has other customs and excise reasons for needing attention. In these latter cases, landfall only at a port of entry.