Folks will tell you that test rides are the way to go. You are near a branch of Leisurelakes bike shop. Think they have bikes that you can try, and they have an open day in March at Cannock Chase where numerous bikes are available for test rides. Tickets are £20 for the day, which could be money well spent.
Also at Cannock chase two weeks after the brilliant leisure lakes demo is bike live; where you can demo bikes from canyon giant and others. After those two events you'll know which bike is best for you.