I assume that you're buying them after the issue date, so the change in their value since that date will be reflected in the buying price.
As an analogy, a company might issue a thousand one pound shares (with the paperwork clearly stating that they're £1 shares). Later on the share price might have risen to £1.50, meaning that anyone who wants to buy those shares will have to pay £1500. However if looks at his paperwork he'll still see that he holds a thousand £1 shares. (i.e. the price of a share on paper is purely nominal and unrelated to the actual price which must be paid to buy such a share, which might be higher or lower).
If you buy 1000 gilts which are nominally worth £100 each on paper, but actually worth £110 each, you'll pay £11,000. If you've only got £10,000 to invest you'll only be able to buy roughly 910 gilts, so the nominal value on the paperwork will be about £91,000.