any good flight schools in the UK or further a field who will offer employment upon completing an instructor rating
I can't think of a school who will automatically offer employment at the end of the instructor course. But it's true that the large majority of instructors hired by a given school did train at that school... In other words, pick a school where you might want to work. And ask about how you will get pay, because some of them only pay you per flying hour.
P