You can only claim for internet usage if the element of business usage
adds to the cost of your internet access.
So if, without the business use, you'd be using a service with a 10Gb per month cap but your business forces you to use a service with a 40Gb per month cap, you can claim the difference in price as an allowable expense.
However if you're using an uncapped service (which you'd be using anyway even without the business element) then, even if your business use is nearly 100% of the total, you can't claim anything because there's no additional cost to you:
http://www.hmrc.gov.uk/manuals/eimanual/eim01475.htm