The UK uses the GSM 900/1800 network, which covers the rest of Europe, Australia and New Zealand, but isn't compatible with the North American GSM 1900. Most modern mobiles can function on both networks, but check before you leave home.
The UK's decision to leave the EU (aka 'Brexit') means that British networks will eventually be able to charge whatever they want for roaming on the Continent, and EU network subscribers will have to pay to roaming fees when visiting the UK. However, the UK will remain subject to EU laws for two years after the official Brexit date in March 2019, so the EU roaming agreement should remain in force until at least 2021.
Other international roaming charges can be prohibitively high, and you'll probably find it cheaper to get a UK number. This is easily done by buying a SIM card (from £5 including calling credit) and sticking it in your phone. Your phone may be locked to your home network, however, so you'll have to either get it unlocked, or buy a cheap pay-as-you-go phone along with your SIM card (from £10 including calling credit).
Pay-as-you-go phones can be recharged by buying vouchers from shops.
Dialling to the UK Dial your country's international access code then 44 (the UK country code), then the area code (dropping the first 0) followed by the telephone number.
Dialling from the UK The international access code is 00; dial this, then add the code of the country you wish to dial.
Reverse-charge (Collect) International Call Dial 155 for the operator. It's an expensive option, but not for the caller.
Area Codes in the UK No standard format or length, eg Edinburgh 0131, London 020, Ambleside 015394.
Directory Assistance A host of agencies offer this service – numbers include 118 118, 118 500 and 118 811 – but fees are extortionate (around £6 for a 45-second call); search online for free at www.thephonebook.bt.com.
Mobile Phones Codes usually begin with 07.
Free Calls Numbers starting with 0800 or 0808 are free.
Call Charges Details here: www.gov.uk/call-charges
National Operator 100
International Operator 155