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.