A title such as "5% Treasury Gilt 2020" expresses the nominal yield. In other words, 5% is the yield you will receive if you are able to buy the Gilt at the nominal (issue) price of GBP100.
Of course, you will not be able to buy such a Gilt in today's market for the nominal price of GBP100. It will be trading at a considerable premium and therefore, if you hold it until maturity you will realise a capital loss to offset the relatively high income you have received.
Here is an example. The "8% June 2021 Gilt" has a coupon of 8%. To purchase a GBP100 nominal Gilt in today's market will cost you GBP135.89. Thus, you will pay 135.89 to receive GBP8.00 income annually. This represents a 5.88% yield (8/135.89 = 5.88%). That sounds pretty good. However, if you hold the Gilt until maturity you will only receive GBP100 on redemption and therefore you will experience a capital loss GBP35.89 on each Gilt purchased. When this capital loss is taken into account it means that the 5.88% yield you are receiving as income will be offset by the capital loss so that you have earned the equivalent of 0.757% annually. You can of course sell the Gilt before its 2021 maturity date, however as the maturity date gets closer the market price will get closer to the GBP100 nominal value and you will again face a capital loss. There's no free ride in the markets. 5 year Gilts currently have a redemption yield of about 0.75%, while 10 year Gilts currently have a redemption yield of about 1.15%.
You may also wish to note that buying Gilts in the open market requires a minimum purchase of GBP10,000 nominal value. However, you can purchase small Gilt holdings through the post office.