The disparity is not as significant as you are presuming. A bond's current price and yield can be deceiving if not considered with respect to the bond's maturity.
What you're overlooking is that when a bond matures, you get back the face redemption value of the bond — not the price you originally paid for the bond. And the market price you originally pay for a bond can be higher or lower than the maturity value, depending on prevailing interest rates vs. when the bond was issued.
If these bonds lasted forever, then comparing current yield would be more meaningful. (And if they lasted forever, they'd be called "consols".)
But with these bonds each maturing within a handful of years, and the face redemption value of the bonds being less than their current market prices, what one has to look forward to at maturity is a capital loss on the value of the bond. I'm presuming that what your £123.08 or £108.11 gets at maturity is an even £100.00 redemption value back. (Where I come from, bonds are typically issued with face values of $1000, but are quoted on the basis of $100, i.e. divided by 10.)
Your overall return of a bond purchased and held to maturity will be a combination of the coupon payment interest, plus either a capital gain if the bond was purchased below redemption value, or less a capital loss if the bond is purchased above redemption value.
Back to your example. If you click into each bond's details, you'll see that the first (maturing in 2021) is presently showing a Redemption Yield of 0.8490% while the second (maturing in 2020) is presently showing 0.7050%. These rates factor in the difference in current price and redemption value.
The better comparison of these bonds looks at their yields-to-maturity and time to maturity. So, in that context, the bond with the extra year of maturity having a slightly higher yield-to-maturity is expected. Refer to Wikipedia - Yield curve - Normal yield curve. But I wouldn't call out the difference as a large disparity — it certainly isn't a disparity as proportionately large as between prices of £123.08 and 108.11 or running yields 6.5% and 4.4%.