Mobile telco Three (also known as 3) offers great value for prepaid roaming SIM plans which can be used throughout the UK as well as in Europe, with free data roaming on your smartphone or tablet.

I regularly travel to London, the UK and Europe and Three's pay-as-you-go SIM is a must-have item on my trips.

This guide will help you get up and running with a Three roaming SIM card in London. 

Where to buy your Three SIM card

You can buy a Three SIM from any Three store in the UK, on arrival at Heathrow Airport or online from Australian prepaid SIM supplier Simcorner. The SIMs are multi-size so they fit all phones.

Three's All-in-One 20 starter plan comes with 12GB of data per month, which should be enough for a few weeks, plus 3,000 minutes of talk time within the UK (or 300 minutes when roaming in Europe) and 3,000 texts sent around the world.

If you want buy the SIM card before you fly out from Australia, Simcorner charges $50 for this 12GB/month SIM plus $5 for delivery.

Another option is the vending machine neat the luggage carousels at Heathrow Airport Terminal 3 (the terminal used by for Qantas and Cathay Pacific flights, among others) which charges £30 ($52) for the 12GB card.

However, Three charges just £20 ($35) for the same SIM at any of its UK stores. The Three stores are accustomed to travellers buying a prepaid SIM, especially the London shops, and they’ll help you with setting up the SIM and account unless the store is very busy.

You can also pick up a Three SIM card without any plan – just the card itself – from any Sainsbury's supermarket for £1, and then buy a top-up voucher to set up your chosen monthly plan.

Hotspotting with a Three prepaid SIM

‘Tethering’ is when you use your smartphone to create either a WiFi hotspot or a wired connection to your laptop or tablet.

This is very handy for connecting a laptop to the Internet when there is no public WiFi available.

While Three officially doesn’t allow tethering, I’ve always had tethering work in the UK, although it only occasionally works when roaming in Europe. 

Roaming around Europe

When using a new Three SIM card in a roaming country you should have first used it for a ‘chargeable event’ in the UK – a quick call, SMS or online session will do. Although Three doesn’t always enforce this, if you are transiting from Heathrow to Europe it’s best to activate your Three SIM while at Heathrow Airport with a quick SMS back home so you can be sure that it will roam.

Enable any Roaming setting on your smartphone and also check that roaming is enabled in your online account at three.co.uk (look under the International Settings section).

Three lets you use 12GB of data per month in any of their 71 roaming countries, but limits your roaming to two months a year or checks that the majority of your usage has been in the UK over a period of four months.

Of course, if you’ll be using your Three SIM for just one trip this won’t be an issue.

While roaming you can make voice calls only back to the UK, with the cost of those calls coming out of your 300 minute monthly roaming allowance. Incoming calls are free.

If you want to make voice calls to countries other than UK, even local calls to the European country you’re currently in, you must have credit loaded on your SIM account. You can do that by getting and activating a top-up voucher.

(The alternative is to make Skype voice calls from your phone, provided there’s credit in your Skype account.) 

Roaming around the world

Three’s SIM plans also cover roaming to many countries outside the UK and Europe such as Brazil, Chile, Singapore, Hong Kong and Macau, Indonesia, Vietnam, even New Zealand and the USA.

However, you might find it preferable to buy a local pre-paid SIM card in those countries. It can be easier and cheaper to top up, and your local contacts may prefer to dial a local mobile number instead of one based in the UK.

Topping up your Three prepaid SIM

To top-up your Three SIM plan for voice calls in Europe or after your first month has ended, you can buy a top-up voucher from almost any UK convenience store or supermarket.

The voucher has a one-time-only code to add credit to your account. Call 444 and go through the prompts to top-up your account.

While you can also do an online top-up through your account on the Three.co.uk website, credit cards issued outside the UK aren’t accepted for this, so I suggest buying to-up vouchers over the counter.

If you’re not in the UK and need to top-up your Three SIM, you can buy a ‘virtual’ top-up voucher from mobiletopup.co.uk. I’ve used this service and it works fine – they charge the same rates as Three plus an extra £1.50 service fee.

While the top-up adds credit to your account, you’ll want to convert that credit into what Three calls an ‘Add-on’ to set up a monthly plan.

Fail to do this – leave the top-up sitting in your account as straight credit – and Three will charge much higher rates for data and voice calls.

You can use your account’s credit to ‘buy’ an add-on directly from your smartphone in the UK, or via your online account if you’re overseas. 

Three has several add-on ‘All-in-One’ plans which last for 30 days and include 3,000 minutes of talk within the UK and 300 minutes when roaming, plus 3,000 texts to any number worldwide:

  • All-in-One 10: £10 for 1GB of data
  • All-in-One 15: £15 for 5GB of data
  • All-in-One 20: £20 for 12GB of data
  • All-in-One 25: £25 for 30GB of data
  • All-in-One 35: £35 for unlimited data

Using your Three SIM on your next UK visit

Your Three SIM account will remain active for six months without a top-up.

That means you can use the same SIM for multiple trips to the UK, as long as there’s no more than six months between each use of the SIM.

If you’ll be returning to the UK you can put enough value in your Three account just before you leave so that you can easily convert that value into a plan when you return to the UK, or even do it online before you fly in.

My advice is to register your SIM online at three.co.uk so that you can manage your account and top-up your phone using a ‘virtual' voucher if you’re away from the UK.