When abroad don't use a dongle with a British 3G sim, that's when you incur high charges it might not even work abroad depends on where you are going, using hotel wifi usually incurs a charge from £10-20 per day, check to see if your hotel offers free Internet before you book. Otherwise go and have a coffee at one of the international coffee shops, they offer free Internet. Heathrow wifi is very expensive £5 per hour! use your British dongle here but airports in other countries may or may not charge, it depends on where you go.
You may possibly be able to buy a 3G sim in the country you are visiting, but most 3G sims abroad are contract only or extremely expensive. In the USA for example, you have to provide proof of permanent address to purchase a sim

Also don't leave your 3G phone or tablet on roaming whilst abroad it will drain your balance in less than a blink of an eye

most people only find this out when they either can't connect or come home to an enormous bill if on contract, hence the EU legislation.