I just returned from a 4-day, 3-night trip to the UK, and I'm happy to say I only encountered DCC at the two Marriott properties where I stayed. At each location, I was able to opt-out without hassle, and the reception knew exactly what to do. The full list of card transactions:

Non-DCC Transactions:

• Travelex ATM with Fidelity Visa at LHR (surprised this didn't offer DCC!)
• Heathrow Express
• Nando's
• Fortnum & Mason
• McDonald's
• National Express Ticket Kiosk (for the Hotel Hoppa at the London Heathrow Marriott)

DCC Transactions (both opted out):

• Grosvenor House (Barclay's terminal that presented an offer of 2.99%; press OK for USD, cancel for GBP)
• London Heathrow Marriott (was not in a position to see the terminal, but assume it is the same acquirer as Grosvenor House since the receipt is the same)

In the case of the LHR Marriott, the receptionist asked the gentleman after me about currency choice to which the gentleman replied "pounds, please". Whether or not that was in response to my proactive announcement about my currency preference, I won't know. However, I was unknowingly hit with DCC at this very location in November 2011. This was the last time I ever let it happen to me once I became aware of DCC.

In the case of both hotel receipts, the credit card terminal receipts printed without the DCC verbiage and clearly indicated that the account will be debit in GBP.

On a related note, the Travelex ATM receipt specifically said, "The exchange rate and authorised amount will be determined by your issuer." That's the way I like it.