My understanding is that no capital loss on a QCB is allowable for relief (either CGT or Income tax) and hence when you are buying a bond significantly above par (which is the case of MOST bonds these days) you end up paying a lot of income tax on the coupons you receive, losing some money in the principal and getting no allowance for that. That transforms what is a low yielding instrument in potentially a negative yielding instrument after tax.
Assume an extreme case of a bond with 1 year left to maturity and priced in the market with around 0% yield to maturity. Let's assume it pays a 8% coupon at maturity and hence its price will be around 108%. If you buy that bond you get taxed at your marginal income tax rate in the 8% coupon you get at the end and you lose capital as the price of the bond drops from 108 to 100. The end result is a loss of 40%*£8 = £3.2 (assuming a 40% income tax bracket) in the £108 investment you made, in what should have been a zero yield instrument.
The QCB regime of no CGT is great if you find bonds below par, but can be nasty in these environment where interest rates a super low compared to the coupons of the bonds that have been issued some time ago.
Please let me know if my understanding is wrong.