@dhillon and @HK198433 - I’m also applying via London to go to a wedding, albeit in my case the wedding is in the States, but for various reasons I had to book late with a transit in Toronto. Never would have expected that a transit visa would take over 3 weeks to process! My friends are asking if I can expedite in some way as the flight is on the 18th, but I really don’t think there are any grounds yet. Still, at the time of application the average processing time was 21 or 22 days, and the current trend is not very encouraging...
When I applied it was 21 days too, it’s now up to 27. I did send a case specific enquiry asking if there was a possibility for expediting but had no response, also emailed and had no response. Fingers crossed you get yours in time!