seeker - you should've received their certificates (and nominet.org.uk login info) about two months or so after registering your co.uk names. you better have 110% correct address listed 'coz that's where they will go. the certificates are must have if you are to sell those names meaningfully in the future.
i'm in london at the moment, and when i go about town i'd say that roughly about 70% of ads in public bear .co.uk tagline, and about 30% com. surprisingly, bigger companies and better known brands usually carry co.uk. signatures, while mom & pop companies usually stick with com's. of course there are some exceptions, but overall this is the impression i've been getting.
btw, go daddy sucks when it comes to uk names (as well as some other issues.) so never again for me. besides, enom has much better uk prices.