Before jumping in too deeply to the answer to your question about a travel adaptor with USB plugs, let me ask if you will need to use the adaptor to plug in anything other than other USB charging points. Of course, it's certainly likely that you'd need one for a traditional laptop, but if you're using one of the newer hybrid laptops that can recharge off USB and if your travels don't require jumping regions on the same trip, I've found that an Anker five-port charger, , plus a specific figure-8 cord with outlet plug for my destination (I have US, UK, and EU plugs purchased from http://www.cablewholesale.com--I don't know if there's an equivalent retailer in the UK) has allowed me to travel without using an adaptor for my last several US and international trips. I have thrown one of these (much cheaper purchased in the US), , in my bag, just in case, on my last several international trips but haven't used it as the Anker otherwise takes care of my charging needs in one central package.

Since car adaptors tend to be inexpensive and are getting better all the time, I upgrade those more frequently. Again, Anker: .