Whilst not disagreeing with your argument, I'm not sure it's that simple in all cases. Unlike say a claim over a contract dispute where a sum of money owed to the claimant can be identified through documentation and settling that sum of money and associated costs in recovering it does satisfy the claim, in a libel case if judgement is found in favour of the claimant then the claimant is being financially compensated in a way which almost always is not definitively attributable to an actual financial loss (as if it is then it is far more simple to issue a claim for the loss that can be documented rather than mess about with libel), and with the judge knowing that financially compensating the claimant is also financially sanctioning the defendant by them ordering payment of those damages.
Having to compensate someone £5000 may be life changing money to one person but small change to the next. Therefore awarding the same damages regardless of the defendant's financial status is not actually delivering consistent results in terms of how severely sanctioned the defendant is. Indeed that type of consideration does happen at the other end of the scale, with no judge being likely to award damages which are clearly beyond all ability of the defendant to pay, so should it not work the other way too?
I don't feel definitely sure that it should and would agree with you in the specifics of the case you documented, but I don't think it's as black & white as saying that the defendant's net worth should not be a consideration.