Ronson - the thruxton rear shocks will raise your ride height and quicken up the steering. You can not raise the front end by swapping longer springs into the fork tubes - no matter what spring you manage to put in there, the length of the fork tube assembly will still be the same. All you may do is eliminate the sag which as stated could lead to very dangerous handling.
I have fitted 1997 Honda F3 cartridge forks to the front of my bike. A simple conversion requiring no machining. Use the Triumph lowers, appropriate bushing, dust and fork seals and the rest of the honda guts and tubes which slide right into the stock triple clamps. Thruxtonone's album has a nice picture break down of the parts layout. I did the conversion using his album only. No Triumph or Honda manuals in hand. Traxxion Dynamics helped with the initial fork setup as they did this exact conversion on Ted Cobb's Thruxton Cup series winning bike. The conversion is somewhat tougher on a bonneville that wants to retain stock ride heigth because of interferance with the handle bars being located over the fork tubes. The forks are from a 1997 F3, and are cartridge forks as opposed to the old damping rod style of the stock forks. Cartridge style forks use a shim stack to control damping, the more fork oil pressure on the shim stack the more they flex - kind of a variable rate keeping the forks from becoming too harsh. The stock damping rod forks work by oil flowing through an orfice of fixed size - no variable rate so to speak - the only way to affect this is different fork oil weight and or different spring rates - both of which are somewhat of a compromise. Fork oil is still flowing through a single size orfice and upon a hard impact the forks will become harsh because you can only flow so much liquid through the opening.
I have also upgraded the Honda forks with Race Tech Gold Valves and Race Tech springs. The Honda forks are about 1.3 inches or so longer than the stock units. The combination of longer forks and shocks will cancell each other out keeping the rake/trail somewhat close to stock. My bike is actually lowered, Bonnie size rear shocks and lowered 1 " on the forks. Foot pegs swapped out to regain clearance lost there. Companies could make a longer set of tubes for you and the swap would require a longer spacer or longer springs with stock spacers to complete. Plenty of options for seperating you and your hard earned cash :-D .
At 29 degrees of Rake the bonneville is somewhat chopper like compared to modern day sport bikes.
Plenty of extra forks tube here.