Many places offer "Wi-Fi hotspots". The latest offering is Wi-Fi on London Underground system (over 100 stations, but not tunnels): see https://www.tfl.gov.uk/campaign/station-wifi
Access to commercial Wi-Fi hot-spots is provided by The Cloud and others around London. Most hotels will provide Wi-Fi access (some charge). Free Wi-Fi may also be available in some coffee shops, pubs, fast-food restaurants, shopping malls and at airports, but some are time limited.
There will, of course, be free Wi-Fi at the event venue.
Pay As You Go SIM cards
Most pay-as-you-go (PAYG) SIM cards will require £10 (or more) to activate. Alternatively the cards can be bought with credit already loaded (for example from O2). Note that three sizes of SIM can bought in the UK, namely nano, micro, standard There are various vendors:
- EE has a £15 prepaid SIM for 10 GB of 3G/4G data.
- A special that you can find at EE stores in London.
- EE Store Finder
- Nearest to Barbican (maybe) 142-171 Unit 1-2 Moorgate, LONDON, 020 7374 6177
- Get an empty SIM from a Three store (cost about 1 pound).
- Register My3 online. (Find WiFi first since the SIM is still empty.)
- Connect a debit or credit card, then top up your account credit from the default £10 to the amount you need (see below). Alternatively you may be able to top up your balance via a voucher.
- Buy one of the add-ons with your credit (e.g. "All in one 15" gets you unlimited data for 30 days over 4G and 3G).
The £12 Giff Gaff "goodybag" above gives unlimited data for one month, but does not allow tethering or tablet use (1 GB option does). These SIM cards must be ordered and sent in the post, and only to a UK address.
To buy over the counter, shops like Currys sell SIM cards.
There has been some debate about Giff Gaff service in Central London.