Dhruv, welcome to the forum. I have several Hornby trains as well and as you have found out the British couplers are not at all like the knuckle couplers. I think it is quite unlikely that you would be able to install the Hornby style couplers on the passenger cars, so you may have to resort to installing a knuckle coupler on the loco. There is an alternative though. How about running a head end freight car? Perhaps if you have a suitable Hornby car that you can use as a transition. Keep the British coupler on one end to hook to the loco, and replace the one on the other end with a knuckle. http://www.kadee.com/ sells couplers and coupler boxes that could be glued in place on the transition car.
The Hornby Class 50 loco is fitted with NEM couplings. I have just purchased the Kadee couplers (HO-knuckle - NEM) but am not sure whether it will work. Will find out when they arrive.
What are NEM couplings exactly and how can these be converted to knuckle couplers?
Well, I think that you have found the perfect solution. The NEM coupler box should accept the NEM-Knuckle coupler you purchased as a straight swap. Just pull remove one and install the new. With luck the heights will line up as knuckle couplers don’t tend to stay hooked if they are offset in height too much.
Kadee has a wonderful web site, so check it all out, they have drawings and directions so you know what your getting in to. Kadee will be your best bet!