You normally hear the Blackadder callsigns around EGHH. Mustang operates out of Redhill along with Rocket.
We went through a company name change recently but wanted to keep our old C/S. It took ICAO nearly three months to sort that one out.
One advantage of a company C/S is that ATC tend to realise that you are a pro doing your job and that time is money. They expect you to be able to comply with any clearance they give to help you out, and for you to fly with a high level of competence and ability. I have heard students using flying school C/S on cross country exercises calling major ATC units when I believed that the C/S was allocated to the organisation and that the student as a hirer was not supposed to use the C/S, and in fact would be better using the aircraft reg with the suffix student, giving ATC fair warning that the pilot is inexperienced and may need a fair degree of help and guidance.
One major disadvantage of personalised callsigns, if you F***up everyone remembers and knows it was you!