You make it sound like travelling in Britain guarantees your bike getting nicked. I know London can be problematic, but it's not all that bad!
My Metal Mules only take about 10 seconds to get off, and offer me peace of mind when it comes to leaving the bike and having a wander. At night, if I'm staying in a hotel, as I've said it's quick and easy to take on and off. The roll bag for my tent takes a little longer . . .
As for the bike itself, I use a disc lock and a cycle (sturdy) lock to 'chain' the bike to something solid (railings, etc). By far the best safety feature is my 'ratty', although fairly new, cover. I bought a cheap one without any brand markings that makes what's underneath look worthless. There's lots on the HUBB about the merits of covers - as you say, out of sight and all that - and my rubbish-looking one seems to make the bike fairly invisible. I've been at campsites where, when uncovered, I get loads of interest in my bike, loads of questions, admiring looks, etc. when it's covered, no interest at all. I get ignored! The more worthless you can make it look, the less interest you'll get. As many have said, most theft is opportunism; the less attractive you can make the prospect (either by making it look really secure or worthless) the more chance there is you'll keep the bike.
If someone really wants your bike, it's gone. To this end, I also have GPS tracker fitted. Is some scrote does nick the bike

, I can tell the Police where it is!