I'd check that the difference between your quote with physical security devices is enough of a saving over a quote without them to be worth it. Back when I lived in the UK, I found that the differences in quotes weren't actually substantial. Though YMMV.
I've heard all kinds of nightmare stories about people's bikes being stolen while they popped into a shop for 5 minutes for a chocolate bar and can of pop and insurance companies not paying out because the owner had declared that it would be secured with a chain while unattended and it wasn't.
That's not to say that you shouldn't use security devices, but I'd recommend that you only declare them to the insurance company if you really are prepared to attach them every single time you turn your back from the bike. If declaring them only saves you 50 quid a year, I personally don't think it's worth the hassle.