The best option depends greatly on where you are going. Some countries have very high density of wifi hotspots. Some bars/diners/coffee shops offer free access to customers, others offer a paid for services. Obviously you can research before you go, ask around when you get there (look out for other people with laptops

) or even buy one of those keyring wifi detectors.
Using a UK based 3g service is likely to be insanely expensive when you roam onto other networks. When I was in the USA it was in the region of £3/MB.

Most networks offer international packages, but still... You could try getting an unlocked 3g dongle and then buying a pay as you go SIM for your destination country. But again this is something to research before going - not all countries allow non-residents to purchase SIM cards as easily as in the UK. Also check what mobile infrastructure exists in the destination country, not everywhere has 3g.