Actually it's been allowed since 3 years ago (early 2011), except on BBC channels due to their publicly-funded status. There are Ofcom regulations which need to be complied with, which includes preventing it in certain program types (news, children's TV, etc.) I don't work with TV much so I don't have full details on the tip of my tongue, but anyone interested will find it somewhere within The Ofcom Broadcasting Code [1]
Top Gear are pretty aggressive about not shilling for manufacturers (c.f. their frequent off-color jokes associated with certain brands, rigged Tesla and Leaf reviews, etc.)
Obviously some manufacturers get a preferential treatment (Land Rover!) in order to keep the presenters in character, but given both the show's success without promotional money and the certain scandal and cancellation of the show (BBC especially is definitely not allowed to do that kind of product placement) if the producers were to be paid off I highly doubt there's any product placement going on.
>BBC especially is definitely not allowed to do that kind of product placement //
You've never seen the Graham Norton Show I take it. It's a thinly veiled infomercial - someone's promoting a book, this persons promoting a movie, this one's selling a new single.
When appearing on BBC though, at work, I was not allowed to wear my company uniform tshirt - cause you know not allowed to advertise ...
