This seems to be mostly avoidable by a) not using a Brooks and b) not leaving it locked up in London
If you really have to do both of those on a regular basis, then QR seatpost and take it with you would be the most secure approach. What I do note is that leather saddles are easily identified by the presence of a carrier bag, shower cap or purpose-built waterproof cover. That Tesco bag screams "Brooks!" from across the street. I propose an alternative waterproofing strategy: One of those ill-fitting saddle cushions of the type favoured by (chiefly female) muggles trying to avoid saddle pain. Those say "nasty stock saddle".
Since moving to Birmingham (from London, with London-quality locks), I've become quite complacent about cycle security, simply because I tend to end up locking next to one or more bikes secured with poundshop-quality cable locks. The main issue I come across is lack of anything sensible to lock to - particularly in the city centre - and I tend to solve that one with a Brompton.
As for component theft, I long ago decided that I'd lose fewer cycle computers and water bottles by leaving them on the bike than by bringing them with me for security and dropping/misplacing them. I've never had a permanently-attached light stolen, though I do avoid leaving quick-release battery lights on the bike, because everyone likes a torch to play with, don't they?
Short of securing your bike inside a locker, you're never going to prevent someone really determined from stripping it for parts. But that seems pretty rare. Deter the opportunists, and be prepared to lose the whole bike eventually.
