Usually it will go something like this… Exec “my client has had a brilliant idea…along the lines of latest quiz show/comedy show/ detective show…..” Creative writer “umm, cough, really sorry but that idea is actually in copyright held by the TV company or production company, and they will sue”
Celador have taken companies to court and won for people stealing imaging material and format ideas from "Who Wants to be a Millionaire". Am I sure about? Is that my final answer? Yes it is.
If you really must borrow someone's idea, think very carefully, saying it's a parody doesn’t stop it being a breach of copyright. Using a VO to sell an item is completely different from Jon Culshaw doing Tom Baker in Deadringers, in a comedy show that is a parody of Dr Who. You are using an impression of Tom Baker to sell an item, Tom Baker may not want to endorse that item, he also could have voiced it, so he can sue for loss of earnings and defamation of character if he is against that product. It could get very expensive for you as the writer and the station for broadcasting it.
I am lifting some pieces from Simon Rushton’s Radio Advertising website (http://www.radioadvertising.org.uk/), as Simon says it very well. This site is a must for anyone who want’s some free advice on radio advertising.

Using Famous or Celebrity Voices
The client phones me and tells me he has had an idea. It's using an impression of Richard Wilson with the final line "I don't believe it" I don't believe it, it's the third time that month I have to explain that the radio station and the client could be sued. Richard Wilson is available to do Voice Overs and makes a portion of his living from that kind of work. An impression of him is a form of theft. You could use the genuine article, but he may not do the catch-phrase and is probably sick to death of people asking him! Talk to your writer. A well known personality could add so much to your campaign. Some Celebrities make themselves readily available for radio work, have great agents and are a delight to work with, but there others who you wont get past their agent because they “don’t do radio”.
Some people wont do certain products or services either for personal reasons or because they are already tied to a regionalor national advertiser in the same category, or their tied into a contract, like soap stars usually can’t do commercials or people doing Motoring Programs may not be able to do car ads. Some are just plain busy, and some just can’t do it. Voicing is a skill and not everyone can do it… even some well known actors just can’t perform in front of the radio microphone. Your producer can advise you, find agents, organise sessions, book London Studios where most celebs want to work, or find studios around the country.
You will pay a premium for Celebrity Voices, but it can be a lot less than you think. Think about producing multiple ads at the same time because the more ads you have done at once the more cost effective it can become. Your writer will be able to give you a quote.
When you are presented with ads for a celebrity voice there are certain things which should have been taken into consideration:-
They will be written for that specific person. Don’t expect that the scripts written for Tom Baker can be voiced by Joe Pasquale.
You WILL have to pay a fortune if you want any kind of endorsement. So don’t expect John Doe to say “Hi, this is John Doe and I love Billy Burn’s BMW Garage” You will be expected to pay thousands if the VO even agrees to do it.
The voice must be recognisable. You might not be able to name the person but the voice will be familiar.
Make sure the voice would convey your sales message effectively. Ask the writer why he chose the voice. He should have specific reasons.

Handy Hints
It is illegal to impersonate living people in commercials without their permission. Why impersonate when you can often get the real person!
Even if a famous person agrees to do a commercial they may not be able to "do one of their characters". The copyright may be owned by someone else.
Scripts should be written for that specific person

Here is what the RACC says:
The RACC http://www.racc.co.uk/racc/raccindex.html
Below is taken from the RACC website

Copyright
To be the subject of copyright, literary works have to have a degree of originality. Generally, advertising slogans are too trivial to satisfy the minimum requirements of originality and hence are not the subject of copyright. Slogans can, however, sometimes be the subject of passing off.
The use of other advertiser's advertising slogans is unwise - their use could be seen as unfair, dishonest and denigratory to the relevant advertiser, or misleading to listeners.

Using clips from programmes/films and spoofs of programmes
Agencies and production houses must ensure that they have obtained copyright clearance before including clips from programmes/films owned by other broadcasters.

Spoofs of programmes/films/other advertisements
Care is needed when doing spoofs of TV programmes/films/advertisements - there is a risk of copyright infringement or passing off. The rights owners would have a copyright infringement claim if they could show that the commercial reproduced 'any substantial part' of the scripts from the programme/film. The rights owners would have an arguable passing off claim if they could show that listeners would be likely to assume that the commercial has been authorised or endorsed by celebrities.
To help ensure that the commercial is nothing more than a spoof, make sure that any music is clearly a spoof version, that the voiceovers are not soundalikes of recognisable celebrities/actors and that any words and phrases are pronounced in such a way that they are clearly spoofs from the programme/film.

Using well-known characters and their catchphrases
If well-known characters from programmes/films and their catchphrases are employed in advertisements, agencies and production houses/departments must clear their use with the original creator/writer by seeking written permission.
This area can be a minefield. Agencies, production houses and broadcasters are advised to take legal advice in any area of doubt.

Source: http://www.mediauk.com/