Hi Louise. I booked several tours in Rome for our trip last September (some private tours and some group tours). Some companies required us to pre-pay but some did not. (We experienced the same thing with the accommodations that we booked. Our apartment in Rome required that we pre-pay while others did not.)
I believe that Angel Tours is a reputable company and I would not be put off by their request to pre-pay for the tour. All tour companies limit the number of people allowed on each tour and from their standpoint, they probably get tired of people reserving a spot only to no show and the tour company loses out. In your case, since you are booking a private tour, you are asking a tour guide to blook out a period of time just for you and if you don't show, they have lost the opportunity to make money for that period of time. As another poster suggested, find out what their cancellation/refund policy is in the event that you are not able to take the tour.
Angel Tours conducts a free nightly tour of the Pantheon and we happened to be there one night at the time of the tour. We listened to most of the tour and were impressed with the tour guide & his knowledge. While I did not book any of my other tours with Angel Tours, I definitely felt that they were a reputable company. They seem to be pretty popular. Have a wonderful trip.