They can be bought in denominations of £10, £20, £25, £30, £50 etc. However once they have been redeemed you get credit. This is table in which you can convert your credit to robux. As you can see, you end up with left over credit. The credit can't be used until you have another gift card, that will also leave you with left over credit as it will never add up exactly to the required price!
My son received a £20 gift card for his birthday, he can't get the full value of it now. We don't really want to be left with £4.51 credit, if he gets another £20 gift card he will still be left with £4.02 credit!