Mobile Phones : If have a mobile phone from outside Europe (especially the US or Japan) check if it will work in the UK and check what tariff your network will charge for calling from the UK to other UK numbers or to home. If your phone does not work here, or the rates quoted are very high, you might like to consider hiring a mobile phone in the UK. You can hire mobile phones from around £1-5 per day from Travelex (the currency exchange company) amongst others and collect one in most of the Heathrow terminals (1, 3 and 4) and both Gatwick terminals; a hired UK phone should give you much cheaper calls within the UK. You need to book before you come, online. Check the cost of calls to your home country carefully (and also from your home country to the phone); it will usually be quite expensive, although not as much as the really exorbitant hotel rates. The cheapest option for talking to people back home regularly is for them to call you in your hotel room (they will pay the cost of a standard international call). If you need to call home from the UK, you might consider to get a Lebara Sim card. For example: phone call to France to landline costs 5p/minute or 19p/minute to Mobile phone.
Internet: Broadband internet access is now available in most hotels in London, for a fee, so you might want to consider taking your laptop with you - it will also allow you to access this guide! Alternatively, internet cafes are very popular and many places also offer WiFi broadband access (e.g. Costa Café, McDonald, pubs...).
Skype: This internet service will allow you to call from your laptop or your smartphone at a very low cost. You easily download and set up an account.