The UK is a fantastic place to start any European adventure. Especially if you're looking for a more 'gentle' introduction to foreign culture compared to starting in a country where you can't speak the language or anything. That being said, the UK is such a diverse place that many times you won't understand the English people are speaking!
Local Language: English.
Visa Requirements: Canadians are allowed in the UK for 90 days without a visa.
Costs: The UK, and London is extremely expensive. Budget at least $100 per day.
I haven't spent much time travelling in the UK but from what I have seen, I'd really like to go back. Two or three weeks is a reasonable amount of time to travel from London and then up to Scotland (Edinburgh for example) and get a good look at the major towns and cities along the way. There's seriously so much to see it can be overwhelming. I could easily spend a week or more in London, that is if it wasn't so ridiculously expensive.
Places I would go, based on my own interests: London, Oxford, York, Liverpool, Edinburgh and some other small towns in Scotland. if you only have a few days though, definitely spend them in London, unless you really want to see Scotland too, in which case start in Edinburgh. Thanks to low cost airlines you can get to many cities within the UK for a decent price, and from those cities you can get to several other parts of Europe.
Hotels and hostels in the UK are not cheap - though prices certainly drop when outside of the major cities like London. Definitely book in advance if you're going during the high season, which is really about any time in London.
London is very well connected by flights. There are several airports serving the city, with the main one being London Heathrow. You can get cheap flights on low cost airlines at smaller airports like Gatwick and Stansted.