The T5 hilton have a standard taxi rate for cars they book for you from the hotel to T5. The charge is £10 so if 3 or more are travelling I would get a taxi.
I usually use a taxi from the T4 Hilton to T3, because I usually have an 11:00 AM flight, and am not a morning person. I'll be trying the T5 Hilton later this month, and will surely use the taxi to get to T3 for my departure.