UK London Fresh Lobster sources / seasons?
Plenty of lobsters are caught in the waters off the Scottish Highlands. I have cooked a live lobster while in the Highlands.
You could check here: http://www.simsonsfisheries.co.uk/
You might also look among Chinese fishmonger shops in London. I live in San Francisco, where the Chinese fishmongers have tanks holding various kinds of live fish & seafood.
I think most lobsters are landed in the south west and I'd assume that any decent fishmonger would have them readily available or be easily able to get you one. If that proves difficult, then a quick Google confirms that most of the well known online fish sellers list them.
Google also indicates the season is from the beginning of October to mid March.