By far the cheapest plan in the UK with unlimited data and tethering is the One Plan from mobile phone operator Three. You can get it from BuyMobilePhones for as little as £25 per month but there are three ways to reduce that even more.
Firstly, the online retailer gives you £190 cash back, automatically, which is paid directly into your account and doesn't require you to post or email phone bills.
Secondly, you can resell the phone on the contract, the Samsung i5700 Galaxy White for around £20 on ebay or through one of the many online phone refurbishing companies around which should bring the price to just under £17 per month or a total cost of ownership of £408.
Thirdly, should you go through TopCashBack, you will be able to collect a decent cashback as well, probably around £40. The 24-month One Plan comes with 2000 cross network minutes, 5000 minutes to 3 mobile phones, 5000 texts, all you can eat data, unlimited tethering, unlimited Skype and MSN as well.
Source : BuyMobilePhones