To cover the costs of buying shares on your behalf and running the Plan, you will usually be charged commission on the amount actually used to buy shares for you (rates vary).
You will also have to pay stamp duty reserve tax, currently at the rate of 0.5% of the value of the shares bought. The total of these two costs will be added to the amount of money used to buy shares. If the sum to be invested, including these costs, is not enough to buy one share, no costs will be charged to you.