i have a friend working in the uk and apparently the prices are comparable. he decided to buy his canon lenses there. prob makes sense if you can claim VAT less admin charges. i am thinking there is not much differential cos of the international warranty by mainstream manufacturers as oppose to camera bodies where you have local warranty and grey set. try Jessops. http://www.jessops.com/storefinder.
Yes actually, I forgot, if you can claim VAT back then it is cheaper than Singapore even new, although probably not worth the hassle of bothering. As I said, second hand it's almost certainly cheaper in the UK.