Lower arms will be something like £500 at the garage to fit a pair. Proper parts are £120 each, and it is a 2 or 3 hour job. ARB bushes - quite a bit more, as you're have to drop the subframe and replace the bar. The part is £150 or so. Certainly do both at the same time, as the arms have to come off to remove the subframe.
I would say that I am surprised at the diagnosis. The normal symptoms of what they have discovered is vague steering and very annoying knocks from the front.
For your symptoms I'd be looking at the transverse arms - the back ones - £35 each and an hour to fit, the hub bushes (£10 each and an hour to fit) and dampers. And a proper 4 wheel alignment.
If you have no knocks at the front end (i.e. it is silent when you are driving over bumpy roads), then I'd not worry about the ARB. Also check whether it has recently passed an MOT - if the lower ball joint is materially worn, it would have failed.