You can't have it both ways - complain when someone does work ridiculously cheap, or, offer free advice on something that you would charge fair money for.
Don't know that that's quite a fair comparison. A DIYer may not have the money to pay a professional (though they can afford to redecorate, apparently) and we're trying to help out, on the basis that they do the work and take the risk. This is the sort of job that, quite frankly, I'd rather a customer did DIY if they could.
But, for this money, the work is unlikely to be carried out properly, and
if tax is being evaded then it does indeed annoy me (particularly as the person in question is a civil servant and thus paid out of taxation). On the other hand, if the person charging £30 is planning to stay in business, then either the person is very much doing this as an aside and so is not serious competition, or simply won't be in business for very long.
Though I am getting tired of DIYers asking basic questions that they could have Googled first - after all, I designed and built my first heating system using textbook information and websearches without asking a single question on any forum, so if they can't be bothered, why should I?
Having said all of this, I do take your point. Perhaps we shouldn't be giving too much free advice to people who can afford to engage a professional.