No easy route to big roach. You may catch one by chance on any number of rivers, but generally speaking, if you want to catch them regularly it takes a lot of effort and dedication....
I suggest that rather than ask others for opinion, you may do better to say what you have already done yourself in your quest to catch them. Yes, you have a wonderful river in the Severn to look for them, and if I were in your shoes I would be checking club websites, match results and even joining clubs with Severn stretches of water just to gain knowledge.
I have fished famous stretches of water for months on end, and all over the place with very big roach in mind, and although reasonable successful, the really big fish have passed me by - or I have passed them by!!
You have to decide how much time and effort you want to dedicate to the catching of them, and then make a plan covering a year or two - to begin with............
Good luck, anyway!!