Tricky question, because from a legal standpoint, only Sterling is legal tender in the UK, but you also legally entitled to make trades for goods or services with or what ever you so want, including bitcoins. So as for the tax, in your case it's clearly wealth income, i would say ask HMRC, how tax is liable on a transaction not conducted in sterling.

Perhaps see;

https://en.wikipedia.org/wiki/Pound_sterling#Legal_tender_and_regional_issues

i don't know about paranoid, but they do have to follow the UK FSA, and HMRC regulations, and 3K seemingly appearing out of nowhere?

Bitcoin is a viable currency, but that doens't mean it's lack of regulatory oversight exempts it or its users from territorial laws & customs.

If you transfer 100GBP to an exchange, and subsequently buy bitcoins, on said exchange and transfer them to your bitcoin wallet. All the bank will know is you transferred 100GBP to another account, possibly charging fees for the transfer.

As for blockexplorer, yeah the bitcoin block-chain is a public record of transactions, X btc from wallet address to wallet address, where your wallet address is more comparable to a Swiss numbered account that a UK current account number. Your UK current account number identifies You, your bitcoin addresses identify your transactions, but not You.

Tax Avoidance, your bank is obligated to inform HMRC on some transaction, and i can't be bothered to look it up, but you can,  certainly a  100GBP international transfer is not something they will pass on to HMRC, now if you we're to move and sell 25,000GBP, then they might want to know where you got it, and where your putting it.

Simple answer; Don't Worry so Much, Be Happy  

edit, ooh:  if your speculating, then it might be worth noting i believe if we're to exchange to GBP and deposit more than 3000GBP in your UK account, your obligated and maybe your bank is obligated to inform HMRC, as it is probably taxable income.

Seriously, i sign up to reply to a specific post, which after 7 attempts at an unreadable captcha image to register. I can't, then 3 attempts at a captcha to send a PM. Please, Admins, change the Captcha images, i beg thee.

In other news, Hello all 

Well done, as for the bubble, of course, it's something new that speculators can play with.  Reality, bitcoin will succeed or fail. As for the currency speculators, MT.Gox total vtc volume, is what 10% of total bitcoins in circulation, so it doesn't really matter what bitcoins are worth against the Euro, the Dollar or the Pound, irrelevant in the long run.