OMG, just now on BBC Radio London Vanessa Felz, handing over to Robert Elms for his 10am show asked him about his book, he actually had the nerve to namecheck the crowdfunding publisher, then did so again when Vanessa asked him to say how people could get involved, in other words how to send cash!
I can't see how this is anything other than advertising for his own personal commercial venture. If Robert Elms opened a bookshop would he be allowed to go on the BBC and say "Bob Elms' Books can be found at 24 High Street"? That would not be permitted, and I do not see why this, as an online version of selling, is any different.
The BBC has allowed Jo Good to spruik with impunity her YouTube channel that is full of covert ads, so I suppose the others think they might as well do the same.
ETA: These presenters are paid a wage/fee by the BBC to present each show - to inform, educate and entertain - their shows are not there as a stall from which they can sell their own wares.