It is a mixture. I live near Luton - good for bizz jets and what you say about Gulfstreams is correct, they are probably in the majority but I have seen Citations, and Hawkers, plus surpise of the lot, a Yak-42 using ADS-B.
No doubt things will continue to improve but until it becomes mandatory (if ever) to fit the kit, we will have to make the most of it.
If you are close to an airport, it is possible to connect whats in the list with just a height by using this as a guide against the ILS and radio messages. I am able to tie-up most in-bounds to Luton.
Have fun